In case people confuse let be very clear what Valve goal is. They believe at some point Handhold PC will be a category with a decent amount of customers. weather that in 2 years or 5 years or more. When that day comes they want SteamOS not windows to be the OS of choice. Steam Deck goal is to position them selves so that a possibility. If Steam Deck successful enough or loved enough by the people who do buy it to convince Aya to start offering a SteamOS version of there next Neo and who ever else get in the market to do so Valve will consider Steam Deck a huge success. Regardless of the exact number they sell.
Weather in 5 years steam deck selling million or they selling 0 but millions of portable PC with steamOS running is selling don't matter to valve.