We're getting there! I think the new Valve hardware (especially the Steam machine) will help a lot, and obviously Microsoft has a lot more AI antics planned for the future, which will drive more users away from Windows.
I wouldn't give it "perfectly viable" yet. It's very good, but there are a lot of popular titles that have kernel anti-cheat that make linux a non-starter if you play even one of the titles frequently. If this can be overcome, I would give linux the perfectly viable title.