Being able to see exact number of players playing could be an interesting metric Blizz might not be wanting to share when it comes to WoW, wondering if we'll ever see it on Steam.
Valve hasn't ever taken 40%. Their cut starts at 30% and drops to 20% after some gross sales threshold. Also steam keys have zero cut and half of your total steam platform sales can be keys meaning if you're effective at selling keys you can have an effectively 10% cut.