They *can* do a lot of things. However I don't see them doing another engine once over again for at least another 2 expansions. If they were going to do major MAJOR things under the hood this would have been the time to do it with Luminous Studios getting shuttered because of Forespoken. They would have had everyone that was any good at messing with the Luminous engine available to them to work on XIV for this engine update. However they did switch from DX9 to DX11 and it took them a long time to do so, could happen again i suppose. Also the devs would never switch to Vulkan even though they probably should as it would simplify supporting the Mac version but I'm guessing that isn't in the cards as it would take much longer and more work than going from 11 to 12. The biggest issue i see with them switching to DX12 (and i am NO expert in this so it is a stupidly simplified explanation of things) is memory management becomes the devs problem where as DX11 it is handled by the API itself. Memory management is one of the main factors in how the public perceives a game being "optimized" or not so it puts a lot more pressure on the devs and ....honestly I just don't think the XIV team is good enough to do it, at least not all at once or for a long time. Something like having the launcher have a beta DX12 version for 4 years or something might be in the cards but i dunno.
I get being optimistic about things but I take a more pragmatic approach. I know that it is a big deal for square to end support for consoles. You could argue that they even held on to the Ps3 for too long and many of the engines issues come from the Ps3 having 256MBs of VRAM and 256MBs of system RAM. However with the Ps4 you still have around 12 gigs of total RAM which is about what the normal mid range graphics card has today. I don't see them ending Ps4 support for another 2 expansions. I don't think they have a FSR type thing on Ps4 so i would guess they would be likely to add that to the console (depending on how many people still play on ps4) to extend its support even longer. So yeah, I'd say 9.0 or 10 before the Ps4 goes bye bye.
Ohh and the Xbox one isn't supported, just the X and S. You could make the case that the S is a slightly more powerful version of the XbOne and you would be right but microsoft has a policy that states you have to have your game come out and be supported by both the S and X, soooooooo. In 4+ years time you can blame microsoft if the ps4 and S still have support.