You could also think of it this way: The developers could have simply left the old STF shops in place. Left you your stockpiles of old STF loot even though they won't drop anymore. Then you'd have your "equal purchasing power." Would you like that better?
Presumably, people have these stockpiles of STF loot because they don't want any more of the things they could trade them for. The conversion is giving you something you can buy new things with. Sure, the conversion rate is on the low side if you have large stores of loot, but you can't seriously expect the devs to give you all the new stuff for free just because you hoarded lots of old junk?
You have the option of buying what the loot can buy now. Equipment, dilithium ore, doffs, etc. If you don't want any of that, the loot will get converted and maybe you do want some of the stuff the marks can buy.
EDIT: I do have to agree I don't see any useful reason for locking the conversion crate, either.
Yeah.... I don't get that part about the crate being locked at all.... The rest of the conversion makes sense to me, but not that.