I think people who know Windows system programming are in a good position to make more money. Much more so than *nix, knowledge of Windows internals is an occult specialty (ever try to find out anything about Windows you needed to know? It's an order of magnitude harder than on *nix).
**Hint** Check the VMS manuals... Same thing different name.
I did quite a few ports of code from VMS to NT. Pretty funny how the internals are so similar. Right up until the point you hit the win32 api!
To the OP. I'm a big believer in the big picture. OS's and API's are just details. After a while it's all the same thing.
I make my living in Linux / Sybase / Oracle and still support VMS. I've learned quite a few middleware API's in my career, MQ Series, Tibco, Talarian...
Don't limit yourself, you'll never know where you end up or what you'll be doing. My first job I was doing Oracle work / DBA and they asked me if I would like to do DECTalk. You know those automated systems where you speak into the phone and it gives you back some info you probably didn't ask for
After that I did Compact Disc publishing back when dual diskette drives where king. All because nobody would step up and take it on.
I guess the life lesson is to keep learning and don't ever refuse to do something different.
If you decide to take this path, stay somewhere for a few years and then jump unless your learning new things. When you get much older, the breadth of experience will help you much more than the depth of your experience. You can jump around while your salary is still low. It gets much harder when you start climbing the salary ranks.
Learn OS X and Objective-C. It can't hurt, you'll never know when you might use it. Maybe you'll actually learn something that you can apply elsewhere.
They deserve more money as they have to use a rubbish API

.
God I hate MS API's. They changed yearly (back in the day) and there's about 3 ways to do the same thing, all with different bugs. I don't do much of it anymore though.