It wasn't just Biden. In every country, old men refuse to quit when they should
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Of course that isn't applicable in this example. I believe Biden more or less slipped into another candidacy because there was no real competitor within his party. But the discrepancy in competency is sometimes visible in politics too.
If Biden had declined to run, you would have seen most of them again for primaries this year. It's hard to know who would have won. Most of the candidates that thrilled some people were deeply disliked by others.
It's rare to find a candidate who thrills everybody in the party. Barack Obama managed it, at least for his first term, where nobody really knew him and were able to project anything they wanted.
The interesting part is that Trump thrills only part of his party, while being deeply detested by others. They do, nonetheless, turn out to vote for him.