It can be hard to tell when you're immersed in the tech community, but OS X only has around 7.3% of the overall desktop OS market share[0]. So if Windows 8 took 1% of the market share, it would have 1/7th the share of OS X.
OS X tends to break older apps IIRC. On the other hand, Windows has excellent backwards compatibility. And there's always the very long support period.
New versions of OS X don't break older apps. The opposite is true: new apps are often coded to take advantage of APIs in newer versions of OS X, those will not work on older versions of OS X.
My 2004 PowerPC notebook can't run the most current OS X, and few new apps work on PowerPC Macs. That's why I still use Adobe CS3 on that computer.
My 2009 Intel desktop Mac can run all apps ever released for OS X, even the ones compiled for PowerPC.
About Microsoft: Lots of drivers needed to be rewritten for Vista in order for peripherals to work. Internet Explorer 9 doesn't work on Windows XP and older. Internet Explorer 10 doesn't work on Vista and older. Modern UI apps don't work on Windows 7 and older.
>"OS X tends to break older apps"
>[citation needed]
Apps that dig deep into the OS are always at risk, VMWare Fusion is probably the best example of this. At my previous job there were issues with versions of Adobe CS (3 or 4, I can't really remember) and Snow Leopard.
Apple removed Rosetta from Lion, no PowerPC apps will run on anything after that.
Apple has a horrible backwards compatibility track record, Microsoft is significantly better in this respect.
A minor QuickTime version update halfway into Tiger's lifespan completely broke RollerCoaster Tycoon 3 for a while. It wasn't an OS update, granted, but still irksome. Also, Mountain Lion throws a hissy fit if you try to run a Flash-based app with Gatekeeper enabled (even if you do the right-click->open workaround).
> My 2009 Intel desktop Mac can run all apps ever released for OS X, even the ones compiled for PowerPC.
Your distortion field is so concentrated! You realize the Mac OSX has been out for quite some time, while Windows 8 has barely a few months in the market, right?
I think it's time you clean out your fanboy lense.