Many people who didn't watch the debate, don't regularly watch his public appearances and read internet comments feel the same way you do.
Calls for biden to step down are not significant in the real world, they are baseless epithets repeated on the internet by anonymous strangers or established conservatives.
The concerns of people are being addressed, any claims to the contrary are not based in fact.
"what's he going to be like in his mid-80s"
Not as important to holding the office of the President as people are misapprehending. Biden's not a car salesman, he is administering a country, and he's good at it. He can wear polka dot bow ties for all I care. FDR was in a wheelchair and often couldn't speak at all.
Anyone who believes there's any reason to replace Biden as the presidential candidate at this point is either allowing themself to be fooled by internet trolls or is a troll themselves.