I'm not. But since most new Windows apps run on XP and most new Android apps run on Android 2; I'm curious as to why this cannot ever be the case on OS X or iOS, both of which land you in incompatible trouble if you're using a legacy version.
Maybe, as a non-developer, I'm missing something. Can you enlighten me?
With each update to any platform, new features are introduced to make developing apps for it simpler. Simultaneously, old features are removed for one reason or another (IE, because the feature was the source of security breaches and whoever maintained the platform decided nixing the feature would be the best way of fixing the breach.) If you're a developer looking to support a wide range of versions, you have a (relatively) narrow set of tools at your disposal. You can't use the easiest, newest, features because then users with old versions won't be able to use it, but you also can't use the oldest features because they've been nixed in the new versions and so users with new versions won't be able to use it.
So you have to contort your code to use the features that are neither too old nor too new.
I've developed for Android before - I hate every single Gingerbread user. They all suck and should die. I'd be a millionaire if I got a dollar for every time I found the perfect tool for what I needed in my app and couldn't use it because it was introduced in Honeycomb or Jellybean.
I've also had the same issue developing for OS X. Initially, Battery Status only supported OS X 10.7. As Apple introduced new features, I added support for them. At the same time, users with old Macs (mostly schools) asked me to support older OS's, so I had to go back and rewrite a lot of code to make it work on OS X 10.5 - OS X 10.9.
I've never had that issue when developing for iOS. All updates are free and easy, so most people get them. Those that don't get them tend not to be the kind of people who would buy the apps even if I supported older versions, so I don't bother. I just focus on developing for the latest and greatest versions of iOS.
(Rarely have issues like this when developing Java apps. Sometimes Python 2 - 3 causes me similar pains when writing apps in that language. I've never written a native Windows app so I wouldn't know anything about it... I'd imagine supporting XP - 8 is probably a huge PITA.)