Hey but maybe I can hit something 1 more time.While I agree with the rest, a 20th level Wizard has 1 slot of levels 9 and 8 and multiples of each other level (plus arcane recovery) and (presuming the use of magic items) likely a lot more firepower from a staff or scrolls.
He also has a clone. In a demi-plane. And a simulacrum. And Wish.
Your martials should be just as effective, in other ways (plus items).