I think if Trump loses to a candidate as weak as Biden, then I don't think it bodes well for the future of the Republican Party at all.
If Biden wins, then I expect Kamala to be elected in 2024. The media will make her look like nothing but perfect, and the Republican Party will have a hard time figuring out what direction to take. Plus, if Puerto Rico becomes a state and gets electoral votes. If a Trump Republican can't win, many Republicans don't want feckless neocons either and many would just stay home if that were who the nominee were again. I know I wouldn't be voting.
That's why Trump must win.