I mean, I followed the 2020 primary. Basically everyone who thought they had an outside chance seemed to be running there. I liked Buttigeg and Booker then, because they most fit the Obama mold -- well-spoken, charismatic, unifying rhetoric, credible claims (at the time) that they could take on the establishment, no serious baggage. Realistically, both were closer to Al Gore than Obama in execution, but still.

Booker clearly never had a chance in the primary to begin with, even moreso than Harris. Buttigeg suffered from never finding a strong enough constituency -- he wasn't loud enough to pull from the populist wing (who went to Bernie), but never made a serious play for any of the other major wings. He couldn't do that the way Biden did (by calling in favors with other popular Democrats) because he didn't have the connections, but didn't even try to do it the way Warren did (by advocating specific policies).

Meanwhile, Bernie never really tried to expand his coalition outside the populist and leftist wings (which he basically took all of). It turns out that those groups are not enough to win a democratic primary on their own. He was also perceived as a liability in the general -- remember, Democrats hadn't totally given up on Florida yet, and embracing the term "socialism" is a massive liability there.

Warren was somewhat effective in building a broad coalition, and had a similar strategy to Obama. She maybe could've pulled off that style of insurgent campaign. Her candidacy was doomed because of two factors: first, she is just not charismatic enough to compete with Bernie for the populist and leftist wings, but they are absolutely required for an insurgent campaign. Second, even if she had won the primary, she is a massive liability in the general election for a lot of reasons -- which ensured she was never going to win a primary when the primary concern was stopping the bleeding that Donald Trump was seemingly-deliberately causing.

Once you've ruled out Booker, Buttigeg, Sanders, and Warren, you're basically left with Biden vs. a crowd of other boring people. Is it really surprising that the guy who can barter for endorsements and trade on Obama's legacy managed to beat people like Klobuchar?

Now, you might say it's possible that those people (Booker, Buttigeg, Sanders, Warren) would have all made better candidates than Harris in 2024, and could have succeeded with Biden out of the field. I'd agree with that! I am not trying to argue that none of these people are better than Harris; I'm trying to argue that none of them are what Obama is. They were either trying to do something very different than what he did, or much worse at it. They all also lost significant "outsider" cred by 2024 for various reasons.