Someone's been keeping a list of "things that are going to kill the game" that actually didn't.
They should add level jump potions to that list.
I'm all for it. WoW's leveling potions were expensive, but that was the only real complaint I had. It didn't degrade player skill, botters didn't abuse it, and it didn't really detract from the game at all.
Also, this isn't a "straight to 70" thing. People will still have the entirety of 4.0 to learn how to play their jobs, and level 70 players will very likely have a pretty good understanding of how to play.
The ONLY legitimate downside to this is that new players using the boost will not get to experience the amazing storytelling this game offers. They can watch cutscenes at the inns, but that exempts them from duties, where some of the more profound moments occur. As such, they might not have the same attachment to the characters that those who play through would, and story events in Stormblood might not be as impactful.
EDIT: Also, to the "it should be free!" folks: entitled, much? Paying for the level boost is industry standard. Just about everyone who has a level skipping potion on their game charges for it (I can't think of any examples that don't).