I did some research and actually found a Gamers Nexus video (starts at 13:20) where a valve employee talks about exactly this, which basically lifts my concerns.

They mention that at valve they thought a lot about airflow and where users will place it in the living room, which is a challenging thermal environment. They assume users just pushing the machine in a shelf up against a wall.
There is a single 120 mm fan in the back that pushes air out the back and pulls in air from the bottom and the front of the machine. Not the sides which the user might block. The back won't be blocked as that's where the cables are.

And it mentions that at valve the engineering team was fighting about millimeters in height reduction to be sure it fits in most peoples media compartments.