Of course things like config files change, but unix remains one of the most backwards compatible platforms to build anything on.
What's even more funny is that you use "Unix" instead of "Linux", implying that software written for one Unix would magically work with others (I mean, that would be case if all was backwards compatible, right?). Have you ever written software for more than one Unix, even disregarding historical versions? The most painful concoctions needed to develop for several Unixy OS's (like, say, autotools) exist for the exact reason that they're not compatible at all. While at the same time, I've written code for Win98 almost 15 years ago whose binaries still run, without as much as changing a software setting, on Windows 7 (and probably 8 although I haven't tried) today. (again, there are plenty of things to critisize Windows for, but backward compatibility is not one of them).
"Of course things like config files change"
Yeah, that's like saying "of course a 2012 BMW is different in a few minor ways from a Model T, but essentially they're just machines with engines and wheels." Eh, yeah, sure.
"At least in the Linux world, your knowledge isn't decimated on an update."
Of course, so that's why the time I put into learning how LILO worked is now paying off so greatly! Oh no wait it doesn't, because LILO went the way of the dodo (as far as I can tell, I don't use Linux all that often any more). Wait, all that time I used to spend on manually insmod'ing kernel modules to get my audio to work with the various audio systems, that has paid off! Hmm no, that too is a crapshoot that changes every year or so. Config file format? Different between all programs. Desktop integration? Different desktop systems, who each change how they work every 2 or 3 years. Gfx card drivers? Depends on the brand, make and (if you're lucky) distro. Networking setup? Lol, don't get me started.