Unlike the philosophical argument described in The Dubious Benefits of Porting FOSS to Windows, Bryce makes a more concrete, more pragmatic argument. In particular, the ratio of potential contributors to users on non-free platforms is measurably smaller than the same ratio on free platforms.
I’ve noticed this problem in some of my own projects. There are plenty of users willing to try a piece of software that may or may not work well on a non-free platform, but when it comes time to debug and fix these problems, their motivation goes away.
I’m sympathetic; it’s not fun to try to build and debug software on Windows. I don’t use it. I don’t understand it. I’m not the person you want telling someone how to install any of the free compilers that somehow don’t come bundled with Windows, just so that someone might be able to produce an interesting backtrace.
I’m not sure there’s a good solution, at least without enfranchising users to become contributors — and that seems to require Free platforms.