Not even 980 gtx sli can run The Witcher 3 maxed out at 2160p at 60 fps, so yes, sli is most certainly a requirement.
You don't need to be able to max out a game to enjoy it. I actually prefer it when devs add settings that pretty much no PC today can max out. Means I can come back to the game in 10 years and it'll actually look better.
But just look at the Steam hardware survey; the majority of users actually have fairly modest PCs, so no, SLI is not a requirement for playing the game.
You should read what I wrote again. The 2-3% of Linux users simply do not cut it, it isn't a market large enough, a market by the way which to a large extent is populated by hackers and people of academia such as people doing physics, like me. The majority of those people do not care the slightest bit about gaming, and those that do are already dual booting.
Your last statement is obviously not true, there are people who care about gaming and don't dual boot.
But about your other point that 2-3% isn't enough: you have to also consider the absolute number. In the end, whether a port is profitable depends on how many people buy it, not on how many % that is out of some group. Last I read, Steam had 125 million users, that was last February, and Steam has been growing rather rapidly in recent years. So the fact that the percentage of Linux users is constant actually means that the absolute number of Linux users is increasing. 1% Linux users is then still more than 1.25 million people, likely more at this point. And if you look at sales data, quite often the Linux sales are closer to 2-3%, so Linux users buy more like 2-4 million people.
GOG has, as far as I know, never published numbers, but I suspect they are relatively more popular on Linux because DRM-free, but the absolute numbers are still small. The gamingonlinux.com survey shows GOG at around 20%, and Steam at ~90%, so there are max ~10% Linux gamers exclusively on GOG (or humble).
Of course, it then depends on how much the port actually costs, and the longer it takes, the less they'll make off it because the game gets cheaper. If they are anyway porting to Vulkan, making a Linux version might not be much extra cost. But if they have to tear apart their engine and rebuild it with OpenGL and then throw it away after one game, it's likely not worth it.
I just wished they would make some kind of statement whether it's still planned at all at this point. I don't mind waiting (well, I do, but I can deal with that), but not knowing at all is just frustrating.