All the people who are going to vote for Biden are going to vote for the Biden alternative. Only difference to me is that they will likely be more enthused about the Biden alternative. To me very little likelihood that they won't show up because they are pissed that their guy wasn't the nominee.
I don't think any pontificating by Dems will have ANY bearing on that. Nothing about Biden looking old will be new to anyone, and nothing about anyone else's concern will have much to do with that either.
It's pretty simple - if Biden turns into Pepe, the old and indefatigable defender who everyone loved in the Euro's this year, the press will cover that. If Biden looks old and makes mistakes, the press will cover that.
No need for one to gnash one's teeth about it. Based on this thread, it's not a surprise that some still feel they get the best odds sticking with Biden. I think it's not as good a bet as the alternative.