For me is quite simple:
An item with high markup you should never risk losing in any circumstances PERIOD.
If you have a rare item, you should never need the same item again to upgrade the first one.
To many one of a kind items, and even like a MM of mod fap there are several ingame the risk of potential scamming in the process of lending it to someone to upgrade his weapon.
Does MA really expect people are buying 2 mod faps just to be able to tier upgrade one?
What about items with the unique or ancient label? Or items where there is only dropped one?
This whole thing will only work in favor of scammers or the really rich that can affort 2 mod fap, 2 mod merc, 2 Foeripper etc...
In any way you can't deny that the introduction of the current tier system is really really bad design and lack of knowledge about the game. (At least for the requirements)
Why don't they change it to and L equivalent of the unl weapon? for example, you need a full TT Svempa XT L to upgrade high end BLP weapons.
And a Full TT Geotrek H36 L to upgrade high end laser weapons.
And even for tier2 you need 2 of them, for tier3 you need 3 of them etc...
And then they even can be consumed or put a risc to it of losing it in case the upgrade process fails.
Same for armor, Infiltrator L to upgrade Eon, Lich L to upgrade Angel and above,...
For FAP: Zychion Pro to upgrade high end faps,....
This way you support the crafters (and the miners that supplying the crafters.) it will still be expensive to upgrade since you will lose your ingredients (Profit for MA/FPC) but not that expensive that you can lose the value of a new car in the process.