One of the biggest attractions of Trump was always "he's different", something that also applied to Bernie from the other end of the political spectrum. The Democrats wouldn't have picked up as many Republicans with Bernie, but I think a lot of what Bernie spoke about would have taken some of the disillusioned crowd that Trump attracted. Didn't Bernie get an ovation from a Fox Town Hall crowd during the primaries? When he put his ideas out there, people clearly liked them.
The problem with Biden wanting to go back to "normal" is that "normal" is exactly what led so many voters to Trump, if the Democrats offered an actual alternative, they could have easily chipped away at his support, although 2020 may have been too late, I think 2016 would've been the prime time for Bernie to run.