Your change would render those dungeons into meat grinders for low level characters just to 'fix' it for high level players with skill bloat on their mind.
This would be a good thing to do, and would also be relatively low impact on the experience of low level players since it would not involve droping them into a party of supermen/women who make them feel smaller than the smallest enemy in the dungeon just by being there bearing witness to the awesomeness of a full lvl 70 toolkit scaled relative to the dungeon.
I've leveled alts with a couple of DoM and DoW classes past lvl 12 each along with most of the crafters past level 10 (can do this using the materials from guild vendors) in about 4 hours. It takes no MSQ quests to do and you can all but skip any incidental cut scenes if you are just wanting speed. I agree about the chocobo, but it's structurally built into the game/story that you get permission via your GC choice. That could be redone to allow a single mount of the player's choice to be available as soon as they hit level 10, say, but no other mounts unlock and no flight unlocks unless you complete the normal objectives. I'm not sure why a level 50 would flee an easy FATE, if they can't call their chocobo, then they need to be more wary and not pull the entire fate onto them self.
Seriously though. 4 hours is plenty of time to level quite far. It used to be that you'd level multiple classes for the cross class skills that would help fill out the cross bar. I haven't leveled an Alt since SB changed cross class to cross role skills. You don't need 20+ hours to achieve level 30 or 40. With the XP bonuses and other elements you can level very quickly now. but yeah, you'll miss things like GC membership and chocobo mount until you go back for story, but if you level through say level 30, and go back, you can cakewalk the MSQ and get that chocobo in no time at ll, especially as there is no appreciable block on obtaining it - the number of GC seals needed was heavily nerfed.