I think the evil valet risk isn't real, but this could be part of a chain-of-attack in some scenarios, mainly rental cars.
Like, sure, if you're just going to use it to spy on the user, you could also rent a rental car and leave a recording device under the floormat, or hidden behind the head unit, or whatever.
But if you have an Apple Carplay exploit, where someone tethering their phone to the car can be compromised, renting a car and flashing a malicious OS to exploit the phones of people who come after you could maybe be a real attack. It's kinda hard to get people to otherwise connect to a malicious infotainment system with carplay, so if you have an exploit that requires that, this could be part of it...
Except actually, no, if you have a carplay exploit, just rent the car, and rewire the USB port to go through a flipper zero or whatever and don't bother reflashing the car's software, that's just as easy.
... So yeah, I guess I agree with you, even in the rental car scenario, where this seems like it would be worst, your attacker might as well just hide something in the car instead of flashing the software.