The framerate is already unlocked (60 fps max), only it barely ever reaches the ceiling right now, thanks to the potato CPUs of current consoles. But the Series X will probably easily max the game out at 60 fps even in 4K-Quality mode. Honestly that's one of the biggest reasons I want a Series X. I don't know how high the framerate would get if they completely unlocked it, but since most TVs have a 60 Hz refresh rate, I don't think it would be of much benefit. A fully locked 60 fps will still be awesome to have.
Agree - I'm gonna be happy with 4K at a sustained 60fps. 120 doesn't bother me.