I'm not sure how the fork of xiv's engine differs from the one they made ff15 or forespoken with but the xiv engine is speculated to be the same one they made 16 with as the dev team that made 16 also makes 14. I'm still pissed it make DT over a year late because if they wanted to make a devil may cry clone they should have named it something other than "Final Fantasy"... anyway. If that is true I would say there isn't a whole lot of difference if any in them as they all should have updated tools within the engine itself. Other engines such as frostbite or unreal generally have versions and every so often their tools get updated to that latest version while they are making games. Of course you can have an unreal game using the last supported version of UE3 or UE4 even though the current versions is 5. The **** show that was BF2042 showed us all what happens when your have underdeveloped tools in your engine and you try and make a game on an unstable foundation.

When thinking of dropping support for a platform you have to ask yourself why, and im not understanding why they would drop ps4 because some people want to say "well it would have RT (or another buzz word that doesn't mean a whole lot - like mesh shaders) in it if it didn't support Y platform". Also they wouldn't be working with "older graphics" they would just be using the compressed textures that they are using now. Think of it like having a low and medium preset on a computer, just because you select the new high preset it doesn't make the low or medium ones not exist.

The Ps3 had two 256MB chunks of memory to deal with and the GPU in it wasn't all that powerful. The ps4's weakness is its CPU however SE would have to come to the realization that their game just wouldn't run the new expansion of the game because of it. That is the only reason I think they would end support for the console. Just to add, the base Ps4 still has the same about of graphical power (or there abouts) as a the xbox S.

There are a lot of things that devs can do that help the game look better and don't effect performance at all. Digital Extremes (Warframe) had redone a lot of their engine much like how xiv is doing now and the system requirements didn't go up almost at all while looking much better.

Honestly you want to see the older consoles get dropped then keep pushing for CPU bound things to be put in the game and keep on yelling at SE to put RT in but I think both of those things are just a wasted effort. As long as they are getting a roi on the old consoles that is where the game will stay, or be supported anyway.