What, Office?There's a lot more than just Office.
I'm writing this on a Windows PC with several heavyweight software packages and plenty more lightweight tools installed, all commercial and legally purchased, that are not available on my obvious alternative platform (Linux).
However, I have little use for a heavyweight office suite. For example, most of the text-based documents I create are either plain text files or "serious" work created with more powerful tools than Word.
as a developer of an end-user application for a PC, you still have to very strongly consider writing for Windows whether you like it or not
You're begging the question. The point is that you can develop end-user applications on many other viable platforms today. Some of them, such as web apps, are also accessible from PCs. Others require different devices, but if my expected customer base already has those devices or would be willing to acquire them to use my software, that's not a problem for me.
There are obviously some advantages to writing a native application over, for example, trying to do everything in a web app. However, writing and maintaining native Windows applications is now so unpleasant compared to numerous other options -- almost entirely as a result of Microsoft's choices regarding APIs, installation/update mechanisms, and other system-level functionality -- that the path of least resistance is to use other platforms when you can and write natively for Windows only when you must. That's very bad news for Microsoft.