You're better off doing anything but poetics for desynth. It's a lot faster to earn the 30k in gil per piece of a vended equipment than the 250 or more poetics for a similar piece. I don't know about the cap; keep in mind this assumes that the items will resolve into worthwhile things. A lot of old gear at 60s gave you ampithetre leather.

For relic...most people will quit at trying to get into snowcloak. If they persist, the light step will kill them; many of the best ways to get it no longer pop frequently if at all. I don't think it will be a draw for many people. SE will have to start to do something about teh lack of incentives for old content; I spend 16 min some days just trying to get intro trials as a rdm.