Developers in general do not update their software for eternity just to keep up with OS breakage, at best at some point they'll ask you to move to a newer version (which you may or may not like - it is far from rare for software to devolve in subsequent versions, especially with paid software that often feels the need to amass features), at worst (and yet very common) the software will be abandoned and you'll lose it.
Apple has the power to not break those programs, not the individual developers. If Apple breaks some API or other functionality that 1000 programs rely on then 1000 developers and their users will need to waste time on fixing the breakage. If Apple does not break some API or other functionality then nobody needs to do anything on that front. It is all at the hands of Apple, not anyone else.
And BTW, i think my previous message was misunderstood, as i wrote in another reply, i wouldn't mind at all to have paid upgrades if that would guarantee backwards compatibility - i know it is hard and time consuming. I still consider it extremely valuable for an OS.
You already know why Apple doesn’t do long backwards compatibility. You were talking about developers of your apps, but Apple is a developer too, and faces the same kind of issues.
These days (unlike the old days) MacOS gets regular, free updates. There’s going to be a cost to that somewhere. Macs and iPhones are probably expensive enough as it is (those hardware sales are what’s paying for OS and platform development).