Here's my two ha'porth.
Serif had developed a reputation for creating some terrific and comparatively low-cost workhorse programs for Windows, and made money along the way—no easy task in the constantly shifting world of software development and user desires. When I heard that they were developing Mac-only programs that appeared to target the core of Adobe's business stream, I was intrigued: this was a very bold move for a small software house.
Although I had limited experience—or need—of using Illustrator, I nonetheless downloaded the public beta of Designer and played with it. I was deeply impressed with the attention to detail, and thus when the public beta of Photo was released I eagerly downloaded it. I've been into photography for over 50 years, and I immediately fell in love with Photo. I both admire and despise PS: PS does some exceptionally clever things but I find the interface clunky and unintuitive (YMMV). Photo, whilst the first version was obviously feature-incomplete, was clearly drawing a line in the sand. The more I played with the beta, the more I came back to the word "elegant". Adobe are a $47 billion company, and yet here is a tiny company based in Nottingham that has aimed an arrow straight at Adobe's jugular—truly a David and Goliath situation. And so, gentle reader, we come to the crux: whatever you might think of Apple, they have always been about exquisite attention to detail. Serif have made a bold business decision to deliver a suite of programs that plays to that mantra. The fact that they've been able to do so with a tiny fraction of the budget heavyweights like Adobe can muster—and with that same apparent philosophic attention to detail—is astonishing. So is a Windows version of the Affinity suite technically feasible? Judging by previous commentary, yes. Whether it is a sound business decision can only be determined by the good folks at Serif. It is perhaps ironic that were a Windows version to be developed, it would only come off the back off the work done on the Mac version. I would also argue that Windows users deserve the Affinity suite: I frankly care not about the platform you use, but I do like to see the best tools placed in the hands of as many users as possible.
May I take this opportunity of wishing all the staff and developers at Serif, and everyone here on the forums, a most splendid New Year.