Re: If only!
Since the only reason to run Windows is because you have an app that only runs on Windows, I'm not sure what the point is.
Except that's not true. Many are Windows developers because customers are Windows users, because Windows has the overwhelming share of their market. It therefore makes sense for them to be using Windows for development and it is convenient to do their Linux porting within Windows.
Of course, developers have been able to do that for years by spinning up a Linux VM, but WSL is - and more so with GUI support out of the box - so much easier.
WSL doesn't prevent developers using Linux to develop Linux apps if they want but it offers a very useful option for those who prefer to use a single workstation for cross-platform development.