Well, it's fairly easy for Google, Microsoft, and Apple to figure out who's using their devices for what purpose, if only because they all now track that kind of information. But yeah, they're not exactly sharing.
I really hope that in 10-20 years we do have a double digit Linux userbase. For one, that would mean Linux will have softened its rough edges to the point where I might not want to commit violent acts while using it! For another, it would mean a true third option on the desktop which has been sorely absent.
I also really hope that gaming on Linux becomes viable, for the sake of Linux users. I know I personally hate being forced to use Linux for tasks simply because the tools I need to use were designed for Linux, and there's no Windows alternative. I don't see it being any more fun for people on the other side of the fence, being forced to use Windows or MacOS simply because their preferred platform isn't sufficiently supported.
Which means Vulkan, even with the framerate hit, really does seem to be the next gen API that deserves priority (because let's face it, once again neither will win, and we'll be locked in this recurring DX vs ____ war until the oceans boil). The framerate issue will almost certainly be addressed eventually. The problem, as I understand it, is just how eventually. User acquisition isn't cheap, and Battlescape is, in part, going to be relying on word of mouth from its early access supporters to promote the game. If the first impression people get is that it's sluggish due to Vulkan's rendering issues, that could potentially have a major impact on the game's success.