I think for someone as advanced in age and tired as Biden is, he needs a truly inspiring VP candidate. He stepped up when we needed him and beat Trump, but Biden also needed to be a one term president as a stopgap for the next generation, but like every other Democrat politician, he's hanging on too long, and we'll pay the price.
I get they tried this with Kamala Harris, to some extent, but she has squandered the opportunity. I'd consider myself fairly well informed, above average for sure, and I'd struggle to name 5 things she's done in the last 3 years.
I think this is closer to reality than many people realize.
I mean, the entire red half of the country doesn't like her for being one or more of: female, black, Democrat. So that's half.
Then even among the blue half, she's got a reputation as a fairly moderate dem whose "tough on crime" shtick falls flat with most progressives who are, to one degree or another, ambivalent at best toward police if not outright hostile. For most who watched the primary debates, I think she came across as tone deaf and incredibly fake and forced.
On paper, she may have seemed like an ideal VP pick for Biden, being black and female, presumably helping him with those two demographics. While I have no data to back it up, to me, on the campaign trail, she didn't seem to connect with either of those demographics, and usually just faded into the background, her job seemingly going no deeper than, "Stand there and make Biden look more progressive" even though the two of them were probably the least progressive faces in the primaries.
That said, I think she still has better than even odds of being his running mate again next year.