Being reliant on a single OS permanently nailed to the hardware is no less crazier. I'd like to be able to install another OS on a vulnerable device, it would help tremendously and not only with the security of that specific device.
Now I've got some expensive paperweights that I can't even use as such because every time I see them I have the urge to throw them in the trash can.
Provide a way to unlock the phones and a standard BSP, it should be the law.
If you don't know what to do with it because your security standards are so high, just give it to someone with lower standards then you, or use it for some project that doesn't involve sensitive data. And if security is broken to the core, there is probably some vulnerability you can exploit to root your phone and do whatever you want with it, including installing a custom ROM.
Still, I agree with you on making it mandatory to provide an unlock method, at least for out-of-support phones.
Just silently enlisted into a "Residential VPN" and a background script that checks for the SSID "Iranian Research Facility" every time you turn your wifi on for some reason.
Sure, a thief may pick your lock, but unless he knows there is something valuable in there, he will probably go find a car the owner forgot to lock, it less effort and there are plenty of them, or he may look for more valuable targets.