This is partly due to it being open source and partly due to its small footprint and stability. Code bases come and go, but the core architecture hasn't changed in literally decades.

I guess that depends how you define "core architecture". :) I'd say there are at least three competing core architectures for implementing Unix:

• Monolithic kernel
• Microkernel
• Hybrid kernel

The infamous 1992 Linux is obsolete debate between old hand respected operating system researcher Andrew Tanenbaum and young upstart Linus Torvalds makes interesting reading. I guess it shows that "theoretical (academic) superiority" does not necessarily translate to success in the marketplace.

I think it's fair to say that monolithic kernels still dominate the Unix arena, though Tru64 UNIX is built on top of the Mach microkernel and Mac OS X is built on the XNU Mach/BSD-based hybrid kernel. Though I'd like to see the microkernel based GNU Hurd succeed, sadly that now looks doubtful after more than twenty years of development -- yet another example of the perils associated with "writing new systems from scratch".