FF is not that strong anymore, as seen by this weeks sales. I wouldn't count that to sell units long term in Japan.
KH3 is what....being released on PS4 right after PS5 launches? We have multiple years before we worry about that title.
MGSV I honestly don't know if it will sell or not in Japan, with the more "open world" designs being implemented. Dark horse for me.
FF is still strong. Spinoffs and sequels obviously will sell far less than mainline FF's and FFXIII isn't exactly the highlight of the franchise so it's not the best example.
We've already seen gameplay videos of KH3 a lot sooner than Versus and FFXIII and I expect development to be easier this gen.
There are also other games that did well on the PS3 like Tales whichI didn't mention.
With third party Support I expect the PS4 to do well in Japan like the PS3