A lot of what is holding them back historically WAS the state of pet AI. That is not the case anymore. Seriously, go test Scholar on a dummy and use the fairy actions. Then swap to a low level job or DoH/L job and back to force your fairy to heal while you do. Ranged auto-attacking pets and Bahamut/Phoenix are just as fluid as that. I remember quite clearly mashing Contagion for 6 seconds straight as late as Stormblood, and that’s most people’s ‘favourite’ old iteration for fluidity (I hated it for many of the reasons I dislike Endwalker SMN, but it wasn’t fluid at high ping either). Compared to then you actively have to go out of your way to force ghosting and pet action delays now. Single weaving pet oGCDs is super consistent.
Alas, we can’t use it in the core rotation at all because the avatars effectively take up 3 GCDs, two of which are exclusively on the summon and dismiss animations from Carby AND the avatars, not the attack the avatars use once summoned. Egi Autos also contributed more overall potency per minute than the avatars do while being completely automatic. So pets now get to contribute less numbers for what amounts to nominal gains in burst. Older iterations always had reliable autos and you always had consistent control over those via the pet hotbar, but even the ShB Egi Assault abilities resolved faster than the avatars do, provided you paid attention to Enkindle/Devotion weaving.
Finally, there’s some other issues. Melee pets likely still suck due to fight movement alone. Ranged only get away with it cause of the Demi burst phases. The Reaper tech also hasn’t been tested on a pet before. That absolutely needs work put in to be sure it does in order to keep those avatar animations around. Even if it didn’t I’d rather have a functional pet rotation that works at a level that lets me chain Akh Morn & Deathflare spam than a pet rotation based around locking me out of all pet interactions period. We have the technology. It should be used.