How often do you play in a game where people can already cast 9th level spells?
That said, I'm not of the opinion the wizard is overpowered at 20. I think they are right where they should be as long as you have a DM with at least half-a-spine, but that melee types do need some improvement. With casters you can go straight 20 levels of wizard and rock. There's no melee-class you want to take alone up to level 20, everyone multiclasses to try to grab synergistic abilities that help them be useful...because they, quite simply, are not useful otherwise.