My biggest concern is the future of the game for skilled players.

I can deal with this change as is, sure, but if there are no future changes to end game to allow the addition of complexity or personalization, I think that will be disastrous.
I think if we the developers think outside the box and allow more skill and more power by choice, then we can cater to new and end game players alike which I think is needed. We need to cater to BOTH.
Example with this change in particular: A black mage mastery passive you can toggle on: I want to challenge myself, so give me a toggle to make cast times longer, make astral and umbral have a timer, but make me do more damage because I'm trying harder.
They said they did not want to raise level past 100, perhaps this allows for more customizability and allowing choosing some skills, mastery or passives later rather than only really choosing slightly different gear and Materia.

I don't struggle in general with the timer as it was in group content in general in content I'm comfortable in, to me it's (almost) purely quality of life for over world, solo content, or for prog.
I will mourn that the growing phase people will miss out on.