70% of the Steam users have a PC that's weaker than the Steam Machine[0]. These things will sell out, that's not the issue. The issue is that Valve probably won't have enough power to secure hardware deals to fulfill all the orders, thus limiting the growth of their hardware side.

[0] https://www.techpowerup.com/342970/valve-claims-steam-machin...

Add "in their living room" and the answer is low. But is it a good experience to game in the living room, on the couch? To me that's an open question that is (sadly) very expensive to answer.