Then tell me what is wrong with the logic.
If your party have had the most votes ever for the past three years, even when losing an election due to electoral college, it's believable for it to happen again.
>and he was universally loved by the left
No he wasn't. Go to a leftist site (not neoliberal, actual leftist). They think he's a capitalist war criminal (drone striking in Yemen and Libya) who bailed out banks. They think he's a polished turd. Moderate liberals liked him but isn't wasn't universal at all.
Biden is basically the same again, policy wise. There's no real reason to expect much less turnout for him (all the people who think he's a creep aren't going to switch to 'grab em by the pussy').
>That was only 4 years ago.
Population growth is exponential, not linear. On top of that, leftists hated Hillary far more than Biden, it wasn't just the Republicans who fucking despise her. She was annoying and controversial and far more right wing in terms of economical right.
Biden explicitly said many times, including the last town hall, that he wants to add special funding to the police, so people concerned about defunding the police shouldn't dislike him for that.