There’s more instances of incumbents successfully coming back from a bad first debate than replacing mid-campaign or not running the incumbent and keeping the White House. Just gotta get out and campaign more.
I wish I had your optimism about this but I don't. The margins were already pretty dicey in key states pre-debate and now they're looking like it's Joever. I don't know how you change people's minds after a performance like that. His evident senility is not going to improve over time.