That's a highly subjective opinion, and yours is not more valid than others'
If the thing is a general purpose computer that can run arbitrary software (and I’d be tempted to include anything that can be remotely updated as "software", so this would include Tesla’s firmware), then I should be able to run my software instead. That’s the true test of whether I truly own my computing device.
Before we even get to Free Software, we need documented hardware architectures.
Software is malleable, and Tesla (or hackers) can change it under your feet. You should be able to inspect it and have a say in what it does or whether to accept the next update.
See the recent Philips Hue debacle for an example of when software that's been running well for years suddenly turns on the user.
See recent car recalls where there is a structural problem with the car.
This is not unique to software.
We as users should demand insight into the software running on hardware we own. Otherwise we only partially own it, and if the device is connected to the internet, we can lose ownership any time.
When you buy a house do you enquire into the grade of concrete used in the foundations? Should “we home owners demand insight into the concrete ratios used”.
I wonder if on electrician forums there are folks saying: “wiring should be open source”? Like because you understand something you want more insight, but you’re happy to be in ignorant bliss on every other aspect of a product.
Same with the software, there is a pc and an FSD chip in there. You could definitely muck with it. You just don’t have any details. Is it because you’re a software engineer and not a metallurgist that you think like this?
Open source is a great thing. But it’s not better (morally) than closed source. Open is just a feature. Like Lego vs a solid plastic toy.