Couldn’t that be thwarted by thieves using car covers that were designed and made to act like Faraday cages? Like those bags they sell for cellphones?

This would prevent automatic updates to the software and remote control at the same time.

They could do that, or they could use a 30 year old flatbed tow truck bought at auction for $800

Hell, if you can afford it, just buy a tow truck, the police would probably help you. (I recall a study from many years ago, where people would functionally help a car thief because of how annoying car alarms were)

imagine a kid locked in a toasty car for a whole day before the update gets deployed

It would be the other way around. You wouldn't be able to electronically break in using the zero day after the update.

Then you do the same thing as you would in an unconnected car, break a window.