This argument is sensible, but given that applications outlive both hardware and operating systems, the alternative is users keeping whole operating systems aro

This argument is sensible, but given that applications outlive both hardware and operating systems, the alternative is users keeping whole operating systems around forever without updating them. The Linux kernel is one opposite example. They are pretty hard line about never breaking userspace APIs. It is overcomplicated and full of traps though. For most end users it's still worth it. The reason you took X11 as an example is because it's still around, all modernized APIs that leveraged broken backwards compatibility are long gone by now.