Nobody thinks Biden is an ideal candidate. Most Democrats didn’t want him 4 years ago, but every other candidate dropped out and endorsed him in order to prevent Sanders from getting the nomination. Not that Bernie would be the best candidate either in 2024, given his age; the point is we didn’t want Biden, but we’re stuck with him now.
No sitting president has ever lost their party’s primary. What’s more—and not surprisingly—the ones that have faced significant primary challenges have gone on to lose in the general. If a president wants to run again, the party isn’t going to stop him, and Biden has long since made that decision.