Definitely this, as most single player engines I've seen in modern times aren't really designed with, for instance, large numbers of simultaneous onscreen actors at a time (think Hunts or certain FATEs especially). There's a reason why for instance the original Doom game engine is still heavily preferred for fans of mass slaughter shooter gameplay, it's because the old school 2.5D sprite based design can actually handle the hundreds of monsters and sometimes thousands of projectiles at a time (in fact, just try playing one of the "tons and tons of monsters in a room" experimental pieces like Nuts.WAD on a GL engine like GZDoom ... you'll find that even on a computer 20+ years newer than the thing was made you still can't get a decent framerate to save your life LOL).
Given the graphical level of FFXVI, and for that matter Forspoken, I could expect a disaster if such an engine was brought over verbatim to FF14. (In particular, given the reviews of Forspoken, I could easily imagine long time players suddenly having to scramble to get a PS5 and re-purchase all their XIV licenses for it or else literally have to get a RTX 4080/4090 to get decent speed in crowded areas. Might even make the Dalaran lag from original Wrath of the Lich King look snappy on less. I'd rather not have to replace my RX 6600 this soon ...)