can your mom upgrade windows 7 to windows 10 without you helping her?
i actually do know what's in the rom. and so does. anyone else who wants to check. and you have hundreds of 3rd party people who do. source code is available. you're literally arguing hardware and software should be tied together to the hardware manufacturer. the entire world of computers disagrees.
>can your mom upgrade windows 7 to windows 10 without you helping her?
Quite possibly, yes! I recall Microsoft pushed upgraded to Windows 10 quite aggressively at the beginning to the point it basically scheduled automatic upgrade. That was 5 years ago, I might not recall correctly.
The big difference between iOS/Windows 10 upgrades and installing custom ROM is that in the iOS/Windows 10 case, you click one button (or even let it install automatically!) and it installs itself. It cannot be easier probably. I can expect it was reasonably tested and that it's reasonably safe.
With custom roms, it really needs to be your hobby to do that. For majority of people, it's not viable solution to do that - they lack time and/or knowledge.
Putting Android 10 into Samsung Galaxy S2 is kinda like putting Corvette V8 engine into VW Beetle. It's possible, there are people that did it, but it requires skill and time to do that.
You can't even go to a store and ask them for help to have a custom rom installed. Plus you don't know what's inside that ROM.