The thing is via Asahi Linux you could use it to boot a modified version of MacOS that would allow sideloading in this scenario. The ability for you to still do that is there even if more difficult.

A full locked down boot rom takes that right away altogether unless you can find a bug in the process to jail break it.

This is why locked down hardware is such a problem in my honest opinion.

My point being apple should not have the right of exclusion after they sell the hardware. The owner should. Apple does not have to make the OS easy to side load on because modern copyright is it's own mess. If the boot rom is completely locked down they are violating your property rights.

Further if the owner has access to the keys of their boot ROM they still have ultimate say. Now to take full advantage of that it may not be easy since it's low level code, but you could still take advantage of code some else wrote in such an instance, and considering how popular apple devices someone probably will.

Heck jail breaks exist and a lot people applying them never have written such code themselves. They just want to get out the box apple put them in to customize things and install other software.