Earlier quoted context omitted.
Certain applications that professionals need are only available on Windows. Most games are as well. Even for software with Mac/*nix versions, a person may have quite a bit of money tied up in Windows-only licenses. I'm not commenting on whether pirating Windows is ethical. I'm just pointing out that sometimes you'll have to ditch a lot of software to ditch Windows.
Quite a lot windows-only software works with Wine, .net stuff runs on Mono and there is still ReactOS -- open source OS written from scratch, binary compatible with Windows.
What I mean by this is, while Notepad.exe may run flawlessly, SolidWorks doesn't.