Since we are talking about pony devaluation, I had a thought on G4s. I know it is kind of unrelated to what you were saying about G3s but it is still related, in a sort.
I posit that once the Brony fandom dies out and MLP becomes just an internet fad (for some of those said Bronies) that the market for G4s will fall out, to an extent. Bronies will sell off their collections and won't buy any future releases, ponies will flood the market, and G4s will be worth even less than G3s.
Plus with more collectors than ever, keeping well maintained, mint to nearly mint merchandise, we will have piles of ponies who will be virtually worthless. We pay top dollar for FS AJ, Euro-only brushable releases, etc now but I think that in some time, these same small horses will be worth just a fraction of even their original retail prices.
But then, that is just my opinion.