Well yeah, if Trump isn't the nominee, then clearly the overall prediction is wrong. And if you think Trump isn't going to be the nominee, that is a good reason for disagreeing with Maher.
Do you think Trump won't run or run and lose the nomination? I see no reason at this point to think Trump isn't going to run. Yes, his power is somewhat waning, but he still has a lot of power (and many supporters) in the Republican Party. And if he runs, I don't see how any Republican beats him for the nomination, including DeSantis. DeSantis needs Trump not to run to have a shot (plus get re-elected Governor) in my opinion.
If we assume for a moment though that Trump does get the nomination, then could you admit that Maher's prediction may come true? Also, if it is another person as the Republican nominee, do you believe that Republican would concede the election if he/she loses or follow Trump's example from 2020?