I don't think multiplat would have helped it massively. FF isn't exactly a large brand on either Xbox or Steam.
As people have said it already, this has multiple causes. Some had their nostalgia fix with Remake only and/or were put off by the ending. Maybe less available PS5s vs. PS4s and the higher price play a part too but I think that's a minor factor. The FF userbase is old and generally has disposable income for these games as their frontloadedness attest.
The huge gap of time between the OG and the Remake might have also been detrimental (the game is no longer as relevant as some fans/SE thought) as well as the overall declining quality since FF13.