Earlier quoted context omitted.
80% is really impressive, but another way to state it is that 20% of the games you try won't work. I don't think I'd put up with that if I was a gamer. I guess maybe it's not evenly distributed. Do a higher percentage of the most popular games work?
That 20% is mostly covered by competitive online multiplayer games that use kernel-level anti-cheat systems which will only work on Windows. There's not a whole lot Valve can do about that, other than continuing to push Linux for gaming and hope that it gets popular enough to create an incentive for anti-cheat providers to start targeting Linux as well.
That way Linux gamers can still play with other Linux gamers if they want (and cheaters).
Not an ideal situation but probably better than nothing.