That criticism seemed sensible several months ago, but where we are at right now, does it really seem like MS's App Store is doing any significant threatening? Just looking at http://windows.microsoft.com/en-us/windows-8/apps it seems clear to me that this is not a threat. These are "iphone games", not "PC games".
From what I have seen, the only competitor to Steam that is even a blip on the radar of gamers is Origin. If anything, Valve is pushing into territory that they think Origin cannot enter, not MS.
Do you really care about Valve's reasons? Let's just shut up and let them use their industry muscle to encourage Linux gaming, and in the end we will all win.
I'm not a "real" gamer anyway so none of it really matters to me anyway, I just find the notion of the future of "real" gaming being touch based low-powered "Surface-like" devices to be very silly. Whether the future of PC gaming is on Windows 8 or Linux, it will look very similar to how looks right now: beefy machines chewing up somewhere around half a kilowatt of power, controlled by keyboard/mice and game controllers.
The most change I consider plausible/viable is in display tech. Either high-dpi screens, or next-generation "Rift-like" displays could complement PC gaming. Input and power are here to stay though.
Also I find it bizarre that so many people seem so invested in being sure that Microsoft's App Store presents the largest threat to Steam. It doesn't currently, clearly, and I rather doubt it will in the future. Origin on the other hand does, but nobody here seems to be interested in them. Origin is a real and current threat, not some sort of hypothetical threat. Maybe there just aren't many EA shareholders/employees on HN...
does it really seem like MS's App Store is doing any significant threatening?
Not yet, anyway. Do you think this situation would remain the same if it ever gained a significant market share? The real criticism here is that MS's strategy is a trojan horse.
From what I have seen, the only competitor to Steam that is even a blip on the radar of gamers is Origin. If anything, Valve is pushing into territory that they think Origin cannot enter, not MS.