The cope I'm huffing here is that Trump is too petty to nominate DeSantis, and that Ernst is popular enough with other GOP senators that she get's the pick over Hagerty.
I think she's quietly one of the least bad GOP senators. She's was fairly vocal in support of police reform in 2020, she's big on reducing sexual assault in the military. Honestly I would have been happy if Ernst was the republican Harris promised to have in her cabinet. (all of the things that would be bad about a conservative SecDef are bipartisan anyway)
Also there's the matter of needing a replacement for Ernst, which would be a lot easier to flip the senate seat she wasn't the one holding it.