It's a calculation. I was going to say that the Biden campaign is not under obligation to re-adjudicate the judgment of the majority of the American public that voted for Clinton - but LOL - I just looked it up and he never won a majority of the vote. He was a plurality winner in both elections thanks to Perot; though he almost made it a majority in '96 with 49.2%. Be that irony as it may, I'll stand by the point.
It's really the same reason GOP candidates can't quit Trump, their voters love him. It's easy for us on the other side to say they need to have some backbone and run away from him, but most of them understand if they did that they might as well not run - their voters are where the voters are. We can, and should, blame a generation or two of GOP leaders and strategists for helping to product the current electorate (along with Murdoch of course), but that doesn't change what a GOP candidate faces in the present tense.
But back to the other side, I would note that Hillary won the popular vote still married to Bill. I don't remember her "me too" sector voters lobbying her to divorce him before she ran.
So ultimately no, I don't think Biden is worried about blowback from being on stage with BC. As MB notes, there is a hierarchy of concerns and Bill still has a lot of fans among the people whose top of the memo concerns align for Biden. On the other hand, you won't see him asked to take any other role either.