I agree that's not ideal, and frankly their "you don't change the horse in midstream" is a lame argument, and as useless as the context it was relevant.
To an outsider it seems like democrats (or republicans) didn't seem to have a good grasp of the optics of having these two ancient farts as main candinates.

Sure, biden did a bang-up job during his term in terms of economics and foreign relations, and i know it was just a one debate while being on flu, but imagine someone like Pete Buttigieg in there debating with trump. He would wipe the floor with anyone, yet he too seems to be supporting Biden, as people want a tight ship that support their members.

And it's not like Trump is coherent all the time, or that he is is lying all the time, mocking people and being arrogant, his voters just don't care. I don't want to shame bootlicking as it's their right to have that kink, but that amount of projecting,  inability to self reflect or being self-aware is astounding. I am sure that i would need to live in USA to understand how he got to be the leading candinate in the 

And this might sound bleak, but i think voters are trusting that biden will be replaced by vice president, if something happens to him, and some hoping that will happen.
This was the case in 2019, at my country, when everyone seemed to think that fresh prime minister of leading party should step aside and he did (not, because of health issues), and was replaced by younger one and in result, we got best prime minister in the history of our country. Sure, she did have haters as well, but that's how politics work.