[AKrumbach]

I wish I could take full credit for this, but I'm paraphrasing Andrew Plotkin (developer for the Inform text-adventure programming language):

Steam machines failed because they tried to replace Windows with a worse user experience. In 2015, there were fewer native games and worse compatibility; nobody is going to just rise up and displace Microsoft from their entrenched throne as THE Desktop PC Gaming platform. Alternately, the Steam Deck is essentially inventing a whole new market; if players want a mobile, AAA game platform the only contender right now is a Nintendo Switch (which has an entirely different games library, with limited overlap mostly in "indie" game titles). So now instead of fighting Microsoft where they're strongest, they're fighting a console manufacturer in one place they can expect to be weaker.

Then once the Steam Deck is a widespread commercial success -- and from pre-orders, it looks like that's guaranteed -- Valve will have the foothold to push SteamOS as an equal games development space as Windows desktop. Coupled with the reports that Proton currently runs over 90% of the popular games which don't have a native client, this very well could introduce Linux (albeit indirectly) to the wider market in a way that could eventually challenge Microsoft's comfortable monopoly position.