This might get contentious.

Why are auto combo buttons, like the ones in PvP, exclusive to PvP? Why can't they be brought over to PvE for sequential combos?

I don't think they simplify the gameplay to a significant degree. As far as I know there is no reason at any point why any job would ever want to restart a combo, or use a later part of the combo before the preceding moves. Even so, I think even if these were implemented into PvE, you should still have the option to map them individually if you so choose.

I don't think this would give players some sort of unfair advantage. It would merely reduce the amount of individual keybinds you need, and free up space on your action bar, a thing that was talked about when Stormblood launched, but I don't think was executed thoroughly. It seems strange to me that the devs would express that they wanted to do that, but didn't use their own ingenious solution they'd already implemented into PvP kits.