> Steam decks just don't have the capacity to last very long on the move.

This really depends on the game. A long game of one of the new dooms? Unlikely. Disco Elysium, or something largely 2D? No problem.

> I just don't see how people play SDs without having a power cord nearby, resting them on your knees.

I've done a 3h SD session on a train ride (trains and planes is also where you can't stream games) and... it's ok. Could be more comfortable, but also it's good enough.