Sure, people switch in the other direction. A whole lot more rarely, but I'm sure it happens, especially under pressure from a particular employer or school maybe.
"Adders" are more common--in both directions. Use both platforms as you see fit.
And I wouldn't say Macs lack graphical power across the board--but they lack models that combine high-end graphical power with LOW-end other specs. They lack a gamer-oriented config. You can get a Mac well-equipped for 3D games--but it will have 8 cores and a fancy easy-access case and other things a gamer need not bother with. A Mac Pro is priced just great compared to similar name-brand machines, but it's overkill for gamers in many ways. And until/unless Apple releases a headless mid-range minitower, it's the ONLY Mac that gives gamers a slotted GPU. Meanwhile, OS X gamers would do best with an iMac. They can't upgrade the GPU, but Macs hold their value better than PCs--so they can trade up the WHOLE machine for surprising little cost, if they don't mind the bother of selling the old one.
What you can't do is make your OWN Mac. A home-made PC is no picnic--you get no support, and you have to spend the time it takes. At the same time, it sounds like a very fun project to me! Especially building a Linux box, which I very nearly did (except I couldn't think of a use for it).
And home-made means cheap! That is indeed gold if you are on a budget. I wouldn't blame you at all for building a PC, if you couldn't afford a name-brand machine (Mac or otherwise) able to play certain games you want.
As for your question: how many Mac users have games as their PRIMARY use?
Very few.
Very few PC users too--most probably use Web and email as their PRIMARY use. Games are most often a close second I suspect.
But some do have Windows machines they use only for gaming. (If they've built their own, for instance). And quite a few Mac users probably have a PC (or a Boot Camp partition) that they use only for gaming, while the Mac handles everything else.
And you could certainly say Mac users in general are a rare breed--but that is changing.
WITHIN Mac users, I don't think Mac gamers are rare at all. It seems that quite a few Mac users game very seriously on OS X. I do--it's the main motivator when I upgrade to a new Mac every few years. I have no doubt that many Mac users count games as their main activity--after Internet usage, that is. Certainly enough to keep the Mac game market afloat so far.
So I say, build a gaming PC if you want one, and don't mind dealing with Windows for the sake of obtaining some games you want. And keep your Mac too, and enjoy the best of both worlds.
For my part, gaming is about fun, and dealing with Windows is not fun. I use Windows for work when I have to, which is thankfully very rare. Here's why I choose to game in OS X and avoid Windows gaming:
https://forums.macrumors.com/posts/5467022/
Now, I've been lucky, and the games I want HAVE come to Mac. If I really wanted some Windows-only game badly, I'd have to re-think my options and decide whether Windows is worth it or not.