Why shouldn't someone be allowed to watch the cutscenes on first playthrough?

[Bixby]

If it becomes a problem, FFXIV can increase the noob bonus to get vets to queue.

Simply increasing the bonus will only encourage more of the same. Speed runners gonna speed run, and making it even more worthwhile to do so isn't going to make them slow down.

One idea is to give the new players a pop-up at the end of the dungeon asking if they feel the non-new players were helpful. Or, y'know, word it however you want. If the new players say yes, the (random, not queued together as a party with the new players) vets get significant bonus Myth (or whatever). If not, they don't.