No thanks to it being active only at <50. New players would get used the shorter GCD, reach level 50, then all of a sudden "Hey, why the hell is my GCD longer all of a sudden?" I think a better way to handle it would be one or both of the following:
* Shorten the GCD for everyone regardless of level, and design the rotations around that.
* Front load the pace at which you earn new abilities, so you earn more abilities earlier on (levels 1-50), at the cost of there being a more of an ability drought in the old expansions (levels 50-80), and then add more abilities during Endwalker leveling (levels 80-90).