The short answer is it is fine for Biden to carry on, if the belief is that he is still the best person to beat Trump for all the reasons described in the last 50 pages or so, despite his obvious inability to debate or think on his feet. He's never been known for his speaking ability.They should, but again that is still a one-way street - every US voter is being told they must do this, how absolutely important (vital, even) it is to the health of the nation that they get out and vote to stop Trump. All of their concerns aren't as important as stopping that man from doing what he wants to, which almost everyone should absolutely focus on.
Meanwhile it is fine for Biden to carry on, just as it was fine for RBG to carry on.
I, for one, believe Biden is "with it" enough to still do the job required of the President in the most technical sense. I would prefer a younger, more charismatic option, someone that is a good speaker and more in the Obama mold.
The only reason many of us are wringing our hands over Biden's debate performance isn't b/c I believe he can no longer "do the job" of President, but whether enough people out there think that he can't. There is a lot of "presidenting" that isn't shown on TV.