Personally I don't think turn based would really work in an MMO as you'd be effectively waiting for everyone in the party to "wait their turn" and it would become boring. The problem (IMO) is that there's too much bloat when it comes to abilities/spells - granted, some may like loads of different options but it can also be overwhelming for some.

FFXI (still doing alright after 20+ years) wasn't actually turn based, but it had a much slower pace of combat making it feel more like the pre-FF12 games. You had to build up resources (TP) to unleash a lot of abilities which also resulted in a more relaxed pace.