Oh definitely; and I don't think there is a great deal of users that would argue for the use of snap over flatpak either, so flatpak would be an ideal unified compatibility solution.

Not sure if I've seen a commercial game with a flatpak release; and given the open nature of flatpak, a company (ie: steam) could theoretically implement their own gatekept repository to manage purchases etc...

The main hurdle with adoption is native compatibility with steam; if they started hosting and supporting flatpak installs, the concept would likely stand a better chance. I suppose you could run the whole application sandboxed, which would theoretically sandbox every game installed; but canonical try that, and well... If you search snap steam, you'll see the issues that brings about.