PS5 will be backwards compatible with 99% of PS4 games, so chances are you'll be able to play FFXIV on the PS5 right off the bat.
It wouldn't be any better optimised, but it would be able to utilise the SSD and quicker processing power to cut down load times.

If or When they decided to actually make a PS5 version of FFXIV we don't know, and probably won't be right away. We won't actually see any overall benefit to it until they phase out PS4 support though, which will likely be a couple years after that, whenever it is.