unfortunately there is some sort of Ponzi, but not the way you do describe it. I'm quite sure that MA is aware about it, but were not able to resolve it so far. Let me explain.
When you start you do have nothing, so you are looking for items to have fun. You will most probably start depositing and after some time (years) you will have collected quite some TT in items. When you're still enjoying the game you will start cycling only that amount of TT that you can recover without depositing. So you're on the top of the Ponzi scheme. As a consequence always new players are needed.
Furthermore, as MU is involved we will have quite some inflation, so that finally new players can't effort items. This is when Ponzi collapses.
As mentioned before, MA started addressing it, but didn't resolve it so far. One move was the introduction of Limited items. What we will see next is some kind of constant decay related to playing time like nutrition/food or energy decay or maybe some sort of income taxation. This does not necessarily imply that playing will become more expensive. It might only imply a different balancing scheme.
We do have quite some old-timers over here. Some of them constantly requesting new features. What they are looking for is new fun. If MA starts realizing that new players are the key, then we will see an improved mentoring system with more community interaction. If the game is good, then they do not even need a marketing campaign.
Edit: removed (see facebook
as this was more a joke and I don't like joking