I think you've got the first part exactly right, @GreenTeaLikesMe.
In an ideal world, at an ideal time, Biden should perhaps be gently led off-stage. If only there were some way for a president to be "demoted" or transfered to a merely advisory position. But, given the alternative, I think any rational person will stil vote fr Biden. Biden on his worst day is more knowledgealbe, with better judgment than Trump on his best day - and Biden has people of integrity, whereas Trump has mostly grifters and sycohants.
I'll disagree on one point. I'm not so sure that Boden's decline is more advanced than Trump's. Trump has, for years, been rambling, incoherent, unhinged. People watching the debate didn't think to compare Biden, who perhaps confused a fact, versus Trump who cited no facts at all. Experts who have studied Trump's speech patterns believe there has been a marked decline.
Personally, I think the immediate problem is that Biden needs a different Vice President, but no one agrees with me.