She's a terrible candidate, goes through staff like Kleenex, and believes the rest of the universe is failing to position her for success.

Again, these are not uncommon traits. Many politicians - and most politicians at rarefied levels - are immense egocentrists at best, and typically pretty narcissistic in general. You have to be extremely full of yourself to believe that you should be the leader of the free world. They tend to be prickly and demanding - it's not like the Trump administration had low turnover, for example. And they tend to feel very entitled (again, to use Trump as an example, he tends to believe that everyone should do things for him because he's him).

I won't deny that Harris isn't as good a stump candidate as Reagan or B. Clinton, or even Obama (who was a decent public speaker, while not being the charisma monster that Reagan and Clinton were). She doesn't come across as either gregarious in small scales or a captivating speaker in large ones. But even in the television age, that's not a disqualifying attribute in a candidate. Bushes 41 and 43 weren't charmers or adept public speakers, Biden certainly isn't....and neither really were Nixon or Carter back in the day.

So if you're a Democrat making your dream "wish list" of candidates you might prefer to Biden, she might not be at the top of the list. But neither is she so terrible that you'd pull the ripcord and burn down the party rather than run with her in a pinch. Plus, she probably has one feature that Biden lacks - she's got the right enemies, in that she's probably even more detested by Republicans than even Biden (while not quite approaching Hillary levels). In an era dominated by negative partisanship, being hated by the GOP could be enough to make her more liked by the base, once she's the main target of attacks....

....but again, this is just folderol. Absent a severe medical event, Biden's running and will remain the nominee. The most severe medical events put Harris in the Presidency, where she will be the nominee as the incumbent. Only in the exceedingly unlikely situation of a medical event severe enough to prevent Biden from running, but not so severe that he continues to serve as President, is there even a chance that the 'back-up' nominee would be someone other than Harris - and that's a miniscule probability. Because no one other than second-stringers or no-hopers would try to seize the nomination from her under those circumstances. Anyone who might be a better candidate in 2024 will certainly just wait until 2028, rather than cripple their brand with a half-assed 2024 "emergency back-up" run.