He made two huge mistakes, and one of them might be unrecoverable. He reneged on his pledge to be a 1-term transitional President. This is likely due to the incredible successes he had his first two years in office. He should have looked at George H.W. Bush to see what can happen in the final two years of a term when it comes to re-election opportunities.
But the bigger mistake was not making ending the Trump threat a priority. Had he appointed an AG who recognized the threat the oligarchs posed he could have tied up Trump in courts and in jail far more than he was. It would have made it far more difficult for him to run, but even that may not have been enough as there seems to have been a concerted effort by billionaire wannabe oligarchs to take over the system starting with owning all the media. There's a reason Elon wants to buy TikTok and it's not because he wants to save the platform for millions of people. Trump may not have needed to do much to organize a Presidential run beyond telling these people, "I will do whatever you want and I will be the target of their anger for you, just keep me out of jail". But Biden could have made it far more difficult for them. There is a large section of the Democratic party who do not understand what they are dealing with (hello Democratic leadership, pretty much all of whom are over 70 years old) and another large section who don't care (looking at you far left).
Now, Democrats pretty much have to wait for the next time Republicans blow up the economy again before they get another chance. Republicans have been pretty dependable on doing that. They certainly seem to be gearing up an economic packaged designed to do precisely that. The rich people will still make billions though so it's worth it to them I guess.