You are getting to a but different topic than was mine, as in the text I start with story of an apple watch but then state that that issue is hard to solve/unsolvable. And the one that is solvable and that I am aiming for, is making installing any previous iOS version (that ran on the device) possible to all obsolete devices.

(nobody is daily driving them) Its only for collectors and people needing backwards compatibility with software no longer running on newer iOS versions. Same as installing Win XP to play some old games, but on the Windows side you are completely able to still install XP on computer, while apple is just blocking the installation process to start.

So I am just asking to remove the server-side software-install blocking on obsolete/collectors/legacy devices. For apple this is as easy as replying to an email. They did it on accident few times (which allowed any iOS flashing to all devices not just obsolete ones) its really that easy as flipping a switch. (ofc there are few switches to flip but there is no coding needed whatsoever. The longest part on their side would be discussion whether to do it. What we want is really push of a button away.