If we can get any upgrades from any floor, is there still any reason to go all the way down? Could people do the whole upgrade process by just repeating 1-10?
You still have to have completed floor 50 at least once before, I think. This prevents the people without Heavensward from completing a weapon.

And yes, while you technically can reach 30/30 from F1-10 now, most of the chests will still be duds. You still have the best odds of the final upgrades on F41-50.