I don't disagree. However, I would be happy if the manufacturer of a pocket calculator or digital watch did not prevent me from installing my own system on it.

I guess it is more debatable for a pocket calculator or a digital watch, but IMO when I buy a smartphone, it should not be illegal for me to install whatever I want on it. And the manufacturer should not add technical measures to prevent me from doing it.

That does NOT mean that the manufacturer should implement the features I want (like root access) or go out of their way to give me tools to install my favourite OS. Just that they should not prevent me from doing it (say if I manage to reverse engineer the hardware). Because I own the hardware I buy. At least I should.