These ancient, unsupported apps aren't things like spread sheets and word processors, they're industry specific applications. They're much more expensive to create and update (and buy) because of limited demand, and there's often little choice in what software a company can use.

I've been working on a project to upgrade a old DOS based industry specific app to a "modern" browser app. There are still people using the DOS based version, and our main client for the browser app is still using Windows 2000 and IE 6.