People don’t necessarily run for president to win the nomination. They run to gain name recognition, to establish a team, to raise money they can use for other races. The list of reasons is endless. DeSantis needs to run to establish credibility. If he loses, but not by much, it makes him an ideal pick for VP. Run DeSantis, run!
If he wins, he wins. If he loses, he wins. Either way, we, the public win also.
DeSantis doesn't even need to badmouth Donald Trump. All he needs to do is contrast on his younger age.
I don't expect Trump would return the courtesy though.