Actually, the Shield TV has active cooling. It's very quiet though.
Also, as I said before, all of the current consoles had far more powerful early dev kits than the final versions, though it is possible that all of them were simply downgraded.
I don't think it's beyond the realm of possibility for a handheld to have fans as long as it isn't burning your hands, but it will hurt battery life especially if it's pulling 20w. Gonna need at least a few hours of play time or this thing is gonna bomb hard. Even with that, you'd want to be able to swap batteries easily, and no one wants to have to do that. Or you need a gigantic like 12000 mah battery, which ain't cheap.