I don’t see why you would think that. Someone like Bernie was too radical for the American left to win a primary, no way he would be considered more appealing to America at large in the general.
It’s frustrating because Biden/Harris are too moderate to capture far left voters and too radical to wrestle the center from a populist like Trump. And it seems many disenfranchised voters are content to stay at home because better isn’t good enough. I’m not trying to absolve them of responsibility, but I genuinely think Democrats could make great strides if the left was as good at banding with each other as the right. Instead, the Democrats lost the popular vote for the first time in twenty years.
Honestly, there is a reasonable chance he gets assassinated this term. No one is out of reach from the sword of Damocles.