The people voting for democrats are going to vote for whoever is on the ballot. Same applies for the die-hard republicans. It's the independents who decide the vote and many of them are now considering Trump. I'd argue that to whoever Trump isn't the obvious first choice pretty much anyone else but Biden would be a more compelling candidate. They know what they're going to get with both Trump and Biden. How much worse could they realistically be if we just pick a random dude from the street and nominate them instead?