I can't beleive I have to say this... but if there is even a chance that 1 action from a wizard is effective for 2 rounds isn't that already showing a big discounnect from the fighter?The fighter still pulls ahead with damage at 20th level, it's just not as far ahead as at mid levels. At 20th level you can get spells off in half the round of combat. Assuming save or suck spells and at DC of 19, 50%(or more depending on save stat) of those are going to be saved against by CR 20 creatures and never work at all, and half of the ones that are successful will only be successful for 1 round. So your wizard is only going to be useful in 8, not 16 of the 32 rounds of combat, and then for only 1 round in 4 of those 8 that actually work.
And a DC 19 can be anywhere from a breeze to an almost can't make... with bounded accuracy even CR 20+ can have a +2 or +3 save or two... now it isn't guaranteed to always be able to target the right save (and legendary resist throws more of kink into tryng to figure out what can and can;t work) but to think that the argument that "That 1st level spell might only stop the enemy from attacking for 1 round not many" is at all a thing seems weird to me...
again that is the 1 thing (well one of 2 things cause they can take damage too just not as well as a self healer or barbarian) they excel at doing damage... the problem is that a few big damage spells can catch the wizard up, and even if the wizard NEVER throws a damage spell they have many other ways to circumvent or ignore combat encounters... and outside of combat the fighter chasie doesn't have soemthing wizard doesn'tThe fighter is going to be trucking along dishing out lots of damage in each round.