MP is used as a barrier to Raise spamming.

Without MP, players would be able to easily spam Raises without a concern. Especially RDM whom can generate a bunch of insta-cast Verraises.

Outside of that, Healers have Piety that exists purely to aid with MP. I don't think they feel like redesigning Piety to be another useless stat something else.

Finally, they probably want to keep MP around because its iconic. Like, most FF games had HP and MP. TP is less used in games, especially as a resource to spend in combat.

I think it's a bit of a shame they've just gone the way of simply outright deleting TP and having physical jobs just not have to deal with any resource management outside of their gauges as well as having Casters just do whatever they want so long as they press Lucid every 60 seconds... Takes some of the fun out of actually thinking about your rotation and resource management if favour of "Herp derp press all the skills!"