While I do like the story book idea, I can't imagine letting everyone start at l50, i90, with access to the comparably harder dungeons in Ishgard.
Remember what Yoshi P said - the first 50 levels are a tutorial, slowly easing people into the game, into mechanics, etc.

I can't imagine a completely new player join Longstop, Cutter's Cry, or AV, let alone any of the HM dungeons. There might be a few that would handle it, but it's an insignificant number.

What I would like to see is a slow condensation of the "lower" levels.
This is partially already happening - new players level fast as hell using just MSQ, and get some of the best equips. IMO the journey from 1 to 50 on a new character is smooth sailing and not long at all.
What they could do in the future is to up the exp even more, and remove the "fetch" quests, in the end leaving only main story parts and dungeons. But dungeons have to ALL stay mandatory.