FHCs keep increasing because people keep buying more blocks but most still feel it's worth selling over using them thus flooding the market and lowering the value. Also FHCs not being able to help stack energy probably plays a part. Beyond that warring has been stale and the gym change has made it easier to get to the transition point of FHCs to stat enhancers. There's so many legit reasons FHCs are dropping, the same can't be said for Donor packs.
Not being able to stack and the gyms are far from new, though. Why would it take so long to impact it? Maybe lack of warring has something to do with it, but really, don't you think that the majority of the people using hotel coupons to war are probably using them to train as well? Even if not all, surely enough to absorb enough to keep the item from dropping 15-20% since RESPO. Which is about what DPs have varied (gone up), no?
It's not taking "long to impact" it's just having a long term impact, the changes started right off also kyle is right the cooldown was the biggest one, not sure how I missed that.
Also the people using FHCs to train have always been less than the people that use them to war, source: wars.
Anyways DP prices will never be too high since after a point everyone that can afford them will use them, and beyond that they wont need more, as that happens price drops. Current price is more massively artificial IMO especially cause people wil RL money will take advantage of these prices as a way to "get rich quick" in game.