1. He's not irreparably brain damaged. He has Auditory Processing Disorder. All it means is that sometimes he doesn't process things he hears correctly. The problem is mitigated through closed captioning technology and speech therapy. Unless you think deaf people are unfit for office, he's perfectly capable of serving.
2. Even if he were brain damaged, the people who voted for him have a correct understanding of our system. When you vote for senate, you are voting for an agenda, not an executive. He just has to show up and vote on the issues he supports. If you support the democratic agenda, why on earth would you vote for Oz? I'd vote for someone in a coma as long as the agenda I support is properly voted for.
As for the overall night, Republicans need to look in the mirror and reassess the insane path they're on. This should have been a layup for them. A glorified corpse in the white house, inflation, gas prices, wage stagnation, looming war with a nuclear power, etc...And yet, the majority of the country looked at what they had to offer and said no thanks. Embarrassing. Eventually they need to ask if they are the people living in a bubble.