[jmmv]

BSDs do not break backwards compatibility (at least from what I know from FreeBSD and NetBSD). You _can_ disable backwards compatibility via kernel-level options and by not installing certain distribution sets -- but the default is to remain backwards compatible.

What being a kernel+userland helps with is in implementing features end-to-end: if you have to change how a certain aspect of the network configuration happens, you can do the kernel changes and the userlevel changes in unison. Which explains why you have a unique set of network tools, great documentation, a simple build system that can upgrade the machine...