Experience! Every portable gaming PC and piece of Valve hardware have flopped. This is a "We have tons of money and are still committed to hardware" release, not a success story.
As for Valve hardware, the only clear flop was Steam Machines -- which weren't Valve hardware at all.
I think it's odd to focus purely on the mass market success of Valve's actual hardware (Link, Controller, Index) since these can all be considered experiments by a company that can afford to fail, and since these have all been widely praised as the best-in-category. Clearly they know how to make great hardware; they just haven't done anything mass market yet. Who knows, maybe the disappointment with Nintendo will fuel sales for the Steam Deck.