I don't understand, how, exactly is fast leveling bad for anything aside from the mid-level crafters? The game's content is top-heavy anyway so it's not like there's any mid-level content aside from the Lv15 dungeon (which isn't much) to miss out on. Getting to is not the game.
Being 50 is the game.

That's when you do darkhold, ifrit, stronghold bosses, etc etc.
Maybe if the game had stuff like Garrison, Eco-Warrior and all that jazz like XI did and it began to be neglected (as it did in XI once the majority of the playerbase hit max level) there would be something of a point... but even then, once players passed that content it was ignored. It's going to happen eventually anyway unless they keep content scaling.

Hopefully SE is smart enough to place optional caps on the mid-level content anyway so that even if a player rushes to 50 they can still experience the content as it was meant to be experienced. Even if the content is added after a player has hit the endgame (leveling normally) they can experience the content the way it was meant to be experienced. There is literally no downside to fast leveling in XIV that cannot be corrected by addressing it properly and let me tell you... making the grind take longer isn't the way.