The advent of hard disks, and installable OS environments changed the model. Outside of zOS, and maybe IBMi, there simply aren't any OSs that actually have provided a fairly stable OS API over time. Even windows has decided they don't care about backwards compatibility despite the fact that for most purposes the core win32 API present in win2k provides basically everything needed to host the vast majority of modern applications. Games might be the exception but people forget the directX was an install-able component until MS decided to tie to to the OS with vista).
I can run 25-year-old Windows 3.1 applications on a 32-bit Windows 10 box and they work correctly. If I have upgraded to 64-bit, I am merely constrained to 22-year-old Windows 95 applications.
Then somewhere around when people started to expect Windows on a computer the lifespans got quite a bit shorter, and that seemed to stick into the late 00s. And it's swung back the other direction now... my late 2011 MacBook Pro is good with current Mac OS or current Windows 10 installations, and is only being retired because its soldered-on discrete GPU has started to flake in a way that makes it unusable. (And a replacement motherboard would be stupid expensive.)