Voters have fond memories of Biden - why, I don’t know but they do - Biden has +7 favorables and Trump is underwater. Biden is considered the moderate (regardless of reality - perception is reality) and is more acceptable to Democrats than Sanders.
Sanders was a weaker candidate - easily.
I disagree. Even though Sanders has been in politics for a long time, he has a sort of outsider appeal to a lot of people. There are also a lot of people who are too dumb to know how bad socialism really is and be OK with the fact he is one. It would pull out a lot of people to vote for him who are not enthused about voting for Joe and more likely to stay home now. You are correct about favorability but the real issue would be turn out and the fact the Left (and uninformed) would be much more likely to turn out for Sanders.
In either event, even if you were right, a hypothetical Sanders presidency would be far worse than a hypothetical Biden one though both would be pretty horrible