Re: "enabling manufacturers to lock you out of the devices you bought...."
Of course, but that shows the problem is rather complex. It's a purely academic problem though, since you can already manipulate safety devices on lots of machines. If you did that, the manufacturer obviously isn't at fault. That's not a real problem. So far virtually all customer modifications of IoT devices made them more secure.
BTW user-modified devices are commonly inspected and approved in the car industry. Many countries have some sort of "approval" process by which your car needs to be checked every few years. It it fails, you need to fix those problems or remove it from traffic.