Those OSes were distributed on ROM by necessity, because that was the most cost effective option available. Any modern limitations that prevent running your own software are not just artificial, but actually require additional effort to implement bootloader locking/integrity checks.

Those still allowed you to run your own code.

ROM disassembly books existed for a reason.

Or back to books where the OSes were written in ink. What's the point of this comparison?

I guess, the way Compaq was able to take advantage from IBM.