Is electability a legitimate issue to run on?

A lot of Biden supporters mention electability as a reason to support him. I think it gets into dangerous territory when you are basing your support on second guessing what others will do. You could have the strange case where a candidate who would otherwise have the votes to win, fails to get nominated because people guess wrong as to what others are thinking.

Barring candidates who are extreme outliers, it is best, especially in a multiple-candidate primary, to vote for the candidate who you think is best qualified. Biden may undo what Trump did, but that is not enough. There were genuine reasons for Trump getting elected, and Trump has not made good on most of his promises. A candidate like Sanders or Warren is willing to take on these issues.