How long do you expect upgrades to work before a break in compatibility?
Forever? At least the linux kernel has an explicit goal of never breaking userspace. That doesn't mean other parts of userspace don't break themselves, but the linux kernel has as a development goal to never break binary compatibility for userspace.
It's important to remember that the Kernel is a very small piece of the puzzle that decides whether your 1991 binary works on a system today. The real kicker is whether the glibc versions match, whether X11/Motif will still be around for a while, etc.
But I still admire the kernel for their stance even if it can't solve every compatibility problem.
Forever? At least the linux kernel has an explicit goal of never breaking userspace. That doesn't mean other parts of userspace don't break themselves, but the linux kernel has as a development goal to never break binary compatibility for userspace.
https://lkml.org/lkml/2012/12/23/75