If unlocking an iPhone and running e.g. AOSP on it were feasible, people would be doing it. And you know that. Your argument is disingenuous.
Equating something like this to closed source software is why some people don’t take FOSS seriously.
You might think I was being facetious, but I’m being completely serious: the only way for FOSS to compete is by producing good products and bringing them to market. If FOSS advocates keep trying to fight some software licensing culture war instead of producing good technology, they’re not going to change anyone’s mind. 99.999% of people do not give two shits about a software license, they just want to use a damn phone.
And I'm not a foss advocate, I just want to be able to run software of my choosing and without spyware, as has been the case since the advent of personal computing.
As a side note, legality seems irrelevant to your position. What if a world government mandated optional sideloading + unlocking? Wouldn't you then argue against that law?
I also want to run the software of my choosing. But there’s not a single phone you can completely do that with. Some of this is due to design decisions, some of it is due to corporate lock-in, and some of it is due to regulatory requirements.
I wouldn’t be against a law requiring side loading and unlocking, I would be in favor of it. This only addresses part of the software on a phone, though. There’s a lot of software on a phone beyond user space applications.
But I do think it would be reasonable to put some hurdles to make it difficult to do. There are completely valid reasons to protect the average user from being scammed by malicious software.
To your other point, firmware is another battle entirely and currently has less practical value.