There was a pretty pervasive notion that if Joe Biden had run in 2016 he would have won. We heard for months leading up to it that it was a shame he wasn't running, then heard throughout the whole campaign that it was a shame he wasn't running, then heard for months afterward that it was a shame he didn't run - he would have won. I know a lot of people who repeated this idea or at least discussed it with others.
I think this was a point of comfort for Democrats who were grappling with a traumatic loss because it gave them something to hold on to. They lost because their best bet, Joe Biden, was grieving the loss of his son. It was tragic and understandable and gave them some peace that their supposedly best candidate had to sit out such an important election and that is why they lost. It was like a sports team losing a championship match because their star player got hurt. It sucks to lose, but hey, what can you do?
This notion evolved over time. It went from being "if only Joe Biden ran" to "Joe Biden is our best shot against Trump." Basically looking back to looking forward instead. This was very pervasive too. This is part of why he was so widely seen as a safe choice, a popular choice, and led so many polls for over a year. If Joe Biden ran, we'd win. Go Joe.
I think on some level he must have believed that too. I don't think he would have run if he didn't. I do think he felt some guilt he wasn't there to stop Trump and he might have really believed he could have beat him. Then for years he heard and read that people believed he was the best person to run and the right person to win. Even if he wanted to run personally, there was probably some motivation to run because he had to. Only he could beat Trump. The Chosen Joe.
We are seeing now how that turned out. Nobody really knows anything. We live in chaos and will die in chaos. Understanding is only momentary and then we are thrust, in the next second, back in to madness.