We will not know until after the convention, after Biden is nominated. He could then step aside for medical reasons or whatever, and the party PTB would chose a new team of candidates. If the polls say Biden can't win, they can execute Plan B. I do not think Michelle Obama was ever seriously considered. The primary replacement candidate appeared to be Gavin Newsom. A possible running mate could be somelike one of the Castro brothers or Henry Cuellar from Texas. In 2020, Trump won Texas by ~620,000.
If they chose to go that route, their choice of running mate could be influenced by Trump's choice of running mate.