I’d add that I don’t at all agree with some of the people in this thread, who are on the left end of the spectrum and mainly seem to be hoping that the Democratic Party will select someone further left than Biden because they personally would prefer a further-left candidate. In the American electoral system, voting is FPTP. That means that you tend to wind up with two large, big-tent, fairly centrist parties (which approximate party coalitions in parliamentary systems), and the smart move for each to win general elections is to run a centrist candidate.

A Big Two party can nominate someone out on the fringes, but then they will cede the general election to the other party if the other party runs a centrist candidate.

In fact, a major argument against primaries is that they may tend to choose a suboptimal candidate for the general election, since they tend towards electing candidates towards the center of the political party, and that that a more-winning strategy for a party is to choose someone not at the center of their party’s views, but between that and the center of the general electorate, and that the party members are more-likely to make use of strategic voting than are members of the electorate that votes for their party’s candidate.

I watched a very similar discussion play out on British political forums over the past decade or so. Due to Labour changing some internal party policies that lowered the bar to party membership, party voting changed. Some left-advocacy groups organized a campaign to get people on the left side of the Labour spectrum to become members, to act in the party candidate selection process, and as a result, Jeremy Corbyn – who is on the left end of the Labour spectrum – was chosen as Labour candidate. There were people who were absolutely convinced that running Jeremy Corbyn would be a stupendously winning strategy because they personally were politically closer to Corbyn and couldn’t imagine why anyone else would vote against him. I watched Tony Blair give a talk where he pointed out that unless a political party wins elections, it doesn’t get to have political power, and that while he was a centrist candidate, he actually won elections and that Labour had mostly been out of political power for an awfully large portion of recent British political history. Sure enough, Labour proceeded to run Corbyn twice and were clobbered in two elections. Now they’re back to the comparatively-moderate Keir Starmer and based on polling, are looking at having strong results in the imminent election.