The cost of doing that pales in comparison to what they are making off of iOS. If they can afford to keep developing iTunes for Windows, they can afford to port their other software. For some reason, I don't think Apple will go out of business if they give it a shot.
> How many different types of Windows and Linux configurations are out there?
So target the most prevalent ones. I don't know about all of the various configurations on linux, but it's really not so bad with Windows - just target Windows 7 or higher, and you should be good. Nobody is saying it needs to look nice or even look like a native program; we just want something we can work with, even if it looks like dog shit.