What would the car maker gain from adding a decoy sim?
Okay, nobody is putting in a placebo sim, but in software, we DO have placebo controls. If you flip a switch saying "don't track me", that usually means "track me slightly less". If you delete something, that doesn't mean delete it - that means keep it, but say it's deleted.
If you go through the Windows install, for instance, even if you flip off all the stuff it will tell you "we're still going to do this, just in less circumstances".
What are those circumstances? I don't know. I'm not even sure Microsoft knows.
It is everyone's information that they value, not that one guy who goes to the trouble of killing the radio.
Given that, I hardly think that 'decoy sims' are much of a stretch.
The only reason a decoy sim is going a bit far to believe, is because it wouldn't actually work. It wouldn't actually fool anyone and would just look bad when the first reviewer pointed it out a year before the car is even available for sale. If it weren't for that, we already have countless example proofs that a company will do literally anything if it will work merely 1% more than whatever it costs. Including car makers obfuscating and even flat out lying about their various connections.
What do they get out of it? data & control, same as ever.