You can also argue that those looking for a ff15 game aren't your typical destiny, call of duty gamers. Those who doesn't have a current gen console that are looking for a jrpg are more likely to get a ps4 because jrpg is more associated with playstation than xbox, hence why even when xb360 had a bigger install base, the ps3 version still sold way better in US. If the allocation is gonna be bigger now compare to last gen console, you can't assume those 80% are existing ps4 owners. some of them gonna be new customers. In other words, those looking to upgrade that are also looking for a ff15 type of games are more inclined to get a ps4 especially when ff7 remake will be playable first on ps4. Now december is too big to be decided by one game. Any other low month like march would make a bigger difference than december.
Yeah you have a good point i forgot FF7R is a timed exclusive.