On the contrary, Resident Evil being "more casual appealing" (if you could even say that?) has everything to do with being a better managed franchise. Final Fantasy used to be bigger than Zelda and a more consistent snd bigger seller than RE (which prior to 5, was absolutely terrible in terms of consistency, 4 was basically saved by ports).
Saying that isn't the case now is ... kind of the point?
People are saying the mainline games are declining because of 7R and XVI, not because of an almost 10 year old game. But even then, Final Fantasy barely meeting the sales of a PS1 game from 1997 on more platforms than ever before and with digital distribution is not really franchise growth.
Every Resident Evil game since 7 will end up being a 10m + seller, except maybe REmake 3. They keep selling faster than the previous mainline games since 7, and they have much better legs than FF games. It's not as if people are pooling together the sales of RE games to come up with that figure. RE already has 2 more 10m+ sellers in the time since XV launched, and they're only selling faster.
There is really no world where FF singleplayer sales numbers look impressive post XV. Especially given the software boom of last gen. FF is no doubt still strong thanks in large part to XIV, but we're talking about its singleplayer relevance which is definitely crumbling.