Alts who “buy” items from themselves to raise prices
Alts who all agree with something universally wrong with the game to sway public opinion
Alts to attack people to try and quiet their voice
People who no longer play spouting false information on purpose.
All the spam accounts that makes forum moderator job more difficult.
There I listed reasons for this being a good thing. CAN ANYONE list a reason that this is not a good idea?
Sure, since you've mentioned five reasons, I'll limit myself to the first five that come to mind.
1) Some Entropians may wish to keep their in-world and forum PMs separate. For example, a healer with a satisfactory number of clients may forego advertising on forums, and wish to channel all healing requests through in-world PMs so they can respond to them without tracking their inboxes on each planet's forum. In general, keeping identity aggregation optional allows individuals to retain more flexibility to tailor their communications to their own unique situations.
2) Certain classes of Entropians, such as pirates and resellers, tend to be the targets of (sometimes-)unprompted personal attacks and claims which, at least in the cases I've observed, immediately fall apart under mild scrutiny (often merely restating a claim stripped of its emotive conjugation and asking why we should believe it is enough). As shallow as these attacks often are, they do still take time to diffuse, and are fairly frequent, and I would not blame an individual for wanting to delimit their identity so they can focus on civil, substantive discourse on forums, and only deal with the drama and
ad hominem attacks in-world. Keeping identity aggregation optional affords Entropians this flexibility.
3) There have been cases of real-world threats in Entropia between feuding PVP societies. The more aggregated online identities become, the more credible such threats will be. Knowing that someone lives near Taco Bell is not personally identifying. Knowing that someone lives in Utah is not personally identifying. Knowing that someone lives in the third story of an apartment building is not personally identifying. Knowing that someone lives near a state boarder is not personally identifying. But put these facts together, and I bet we could pinpoint someone's residence to an alarmingly narrow set of possibilities (I just made all of these up and have no idea if they're actually consistent). Keeping identity aggregation optional affords individuals who have concerns about doxxing or real-world threats an extra layer of obfuscation.
4) In general, a strong baseline inclination toward anonymity and identity disaggregation is a powerful defense against rare social catastrophes. PCF Rule 3.6 will forbid concrete examples, so I'll just note that many historians believe some of the most catastrophic events in modern history could not have occurred without the harvesting of personally identifying census data.
5) In general, the causal relations between anonymity and its benefits are more abstract than the causal relations between anonymity and its costs. When a soda gets stolen from the school refrigerator, the potential benefits of a security camera are immediately apparent. When a teacher encourages (and genuinely wants) her students to ask all of their questions, but students, especially the most confused ones, are incentivized to filter the number and character of their questions due to the game theoretic implications on future grading of their signaling too much of their confusion to the teacher, the educational costs of not having an anonymous medium for questions are far less visible.
An implication of the conjunction of 4) and 5) is that anonymity is a crucial tool, perhaps the only real tool we have, to guard against the formation of negative black swan events. Black swans are nearly impossible to predict, emerging out of the unknown unknowns of human knowledge, and have paradigm-shifting impact on the course of historical development. Obviously, the proposal under consideration is just one tiny puzzle piece in the overall trend toward mandatory identification and identity aggregation on the Internet. The problem is that this is a Sorites Paradox; just as one cannot discern which grain of sand's removal would annihilate the pile, one cannot discern which anonymity-sacrificing proposal would cross the line into a state of unacceptably high propensity for catastrophic black swans. Trying to amortize the expected costs of all such possible black swans over all possible sets of anonymity-sacrificing proposals is pretty clearly a non-starter. Linear cause-effect thinking at the event level strongly biases cost-benefit analyses against anonymity, and we should incorporate systems thinking into our toolbox to take cognizance of vitally important, but rarer and less apparent forms of impact. If we fail to do so, then we fall victim to the streetlight effect, searching for answers only where the causal chain links are well-illuminated. "The first step is to measure whatever can be easily measured. The second step is to disregard that which can't easily be measured or given a quantitative value. The third step is to presume that what can't be measured easily really isn't important. The fourth step is to say that what can't be easily measured really doesn't exist. This is suicide." It may be that even if we don't take online privacy to be an inalienable right, treating it functionally as such, or at least giving it very heavy weight as a baseline for evaluating the cost-benefit analyses of individual anonymity-sacrificing proposals, is the only reasonable heuristic to prevent catastrophic events which are too rare and/or too causally opaque to effectively analyze without taking special precautions.
As for the five reasons given in favor of the proposal under consideration, I would classify at least three as largely sunk costs, and the other two as likely net negatively affected by the proposal.
It is probably not wise to try to derive much information about the value of an item from just a few forum sales, even if they're legitimate, and it is difficult to post more than a few illegitimate forum sales for a single item without detection. Folks should initially focus on an item's fundamentals to estimate its value, and incorporate the technicals after it has enough known price points that the law of large numbers has a chance to aggregate out the subjective factors of price (seller needs the money to go on vacation, etc.). The "when an item transacts once that's its value" mentality is pragmatically bankrupt almost independent of the transaction's legitimacy.
Similarly, I'd grade a notion of "public opinion" based in anything other than the merits of the collective cost-benefit analyses produced by public discourse as worse than useless. Every forum poll is grounded in convenience sampling, which as a form of nonprobability sampling, cannot be statistically generalized, and the framing is often loaded in such a way that misses the most important question and/or excludes the best answer. Trying to take a temperature of sentiment by counting posts strongly biases conclusions toward simplistic opinions that are easy to express rather than opinions that take time and effort to flesh out and align effectively with the broader vision of Entropia. A naïve democratic notion of "public opinion" may be of limited use at any rate. Game design proposals should stand or fall on the merits of their reasons; popularity isn't a good first principle.
Anyone attacking forum members to the extent of violating forum rules would already be liable to moderation, a line I expect would come before chilling effects on speech. You may be alluding to the personal attacks against pirates and resellers mentioned above, which
have occasionally caused unwarranted harm before being deleted, but these have typically been launched by forum members who don't appear to be alts, and forced identity aggregation would presumably have a net negative impact for preventing these attacks.
I don't know that there's an intrinsic problem with ex-players being forum members. As for the general issue of false information, this comes back to evaluating claims on the merits of their reasons, not merely on the fact that someone said a thing. This bar seems like a wholly sunk cost for anyone who uses the Internet. "If it's on the Internet, then it must be true" is a meme, not a dream.
The spam accounts seem to have subsided lately, and I can't imagine any saved time would negate the overhead of verifying in-world accounts. In any case, community moderation is almost free. I actually find it quite odd (and possibly bad optics) that we haven't recruited any new player moderators since the others have become inactive, and have gradually started to see more active moderation from developers.
A lot of these concerns are newer issues that weren't really prevalent, i.e., ten years ago. Obviously some alts are to be expected, since folks will always find incentives to create them, but the undue emphasis on tiny price histories, polls as a serious means of data collection, public opinion as a first principle, drama of interpersonal attacks, unsubstantiated claims, and a general myopic atmosphere of uncritically accepting the intuitive, "two inches in front of one's nose" narrative are to a large extent, I think, sociological reverberations of our modern
de facto game design trajectory, ultimately stemming from, among other obstacles, a severe misalignment between Entropia's fundamental value proposition and the incentive structures imposed on planet partners by an Entropia Platform model in vital need of reform, not characteristics of the fundamental nature of the Entropian or the Entropia community.