It's a small group when you compare it to, say, the number of mobile users. On the other hand, not so small that they didn't bother coming out with the iMac Pro. Also not so small that they haven't come out with a new "behind the Mac" ad campaign targeting those users.
So my personal opinion is that the Mac Pro line won't be going away any time soon. In its current form it's not gamer-friendly, and like you I don't see signs that Apple will change that. Sigh. Eventually the world may decide that the idea of a PC isn't necessary anymore, but I don't think we're close to that yet. Ask me again in 5 years..