I've been using Macs for more than a decade but to my mind, the build quality decreased a lot. After my MBP 2011 failed right after the warranty expired it took Apple more than a year to acknowledge that the machine (and lots of other MBP from 2011) were flawed by design and replaced the main logic board for free. Since I needed something to work with while waiting for Apple to respond, I gave Linux anther chance and I have to say that Linux on the desktop is really easy to use. In fact, it worked so well, that I sold my MBP after the repair.
I recently tried Windows 10 for a couple of months but finally decided to switch back to linux since in my experience, I got a much better user experience in every aspect.
What Linux is missing are great graphic tools and since Adobe does not seem to be interested I actually see this as an opportunity for Serif.
Two years ago a figure of $500.000 was mentioned that would be required to fund the development. I'm not sure whether this figure is still acurate given the fact that a Windows port is now in beta -(I'd assume that a lot of the code is already cross-platform). But even with the $500.000: you'd just need approx. 5.000 paying customers when you would charge $99 per copy (or 10.205 customers when sticking with the $49 price tag). I'm pretty sure there are enough people who would gladly pay $100 for Affinity Photo if they could avoid having to use Windows.
I really don't think that these numbers are totally unrealistic but you don't even have to guess or estimate anything here - just setup a kickstarter campaign with a goal of $500.000 and then let's see if there is enough interest in the matter.