We've never had a pair of candidates this unpopular. (Maybe Trump vs Clinton)
If the Dems had any kind of brains, they absolutely would pick someone else at the convention.
Just not Harris; She might be the only one, less popular than Biden is.
Trump has his evangelicals, they're a done deal. His others voters are a "tear down the system" vote. People who are tired of being fucked over, no matter who's in office. They see him as the "worse" things need to get, before they can get better; And they may be right.
If the Dems picked a genuine progressive, who offered several big radical changes (that we all know the ownership class hates), they would take a lot of votes from trump. They would win.
But I doubt they'd do it.
The only thing more scary to Dems than Trump, is a genuine progressive.