My groups routinely does fights without in combat healing on high power campaigns without much trouble. Small amounts of D8s+a little bit simply can't keep up with level appropriate monster damage. And for emergencies, Delay Death(which is an immediate action, and past a certain level of power all your beatsticks should have Beastland Ferocity cast precombat anyway) handles it just fine. Same for Revivify/Last Breath.
But he has tons more ways to prevent the enemies from hitting him compared to a fighter.Except survive when hit for a reasonable amount of damage - especially wizards even with good con.
I actually believe a 10th level wizard is tons more useful than a 20th level fighter. An 11th level wizard even more so.Would you agree that a tenth level wizard is still useful next to a twentieth level fighter or fifteenth level bard?
Same goes for bard. They both get 5th level spells, except wizard spell list>bard spell list. Although a 15th level bard can stand next to a 10th level wizard and not feel useless.