Biden withdrew from the 1988 race. His 2008 performance was so bad, he dropped out right after the Iowa caucus results: he was fifth place. But he went on to earn the biggest popular vote victory in American history and defeat Donald Trump. Conditions are now much more conducive for Kamala Harris.

Look back at candidates who made a primary run, got beat, then came back next election cycle. It’s pretty common

I'd argue that Biden had much higher name recognition in 2020 than Harris has now. And dropping out after the Iowa caucus is actually much better than what Harris managed to do, since she never actually made it to the primaries – her polling tanked before that. (Popular vote is also irrelevant.)