I don't want to dox myself over this, but the Verge review for the steam deck is starts with "it's not ready". The Verge review for our product starts with how easy it was to setup and how impressed they were with the UI (which has to deal with constrained input just like the Deck)
While they're not direct competitors, I leave you to guess which is more approachable to the average user.
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Valve clearly did not put in nearly as much effort into the polish of the frontend software as they did with the hardware and lower level stuff.
Hardware wise the thing is a marvel, and Proton is an engineering marvel... but then a top post on the subreddit is complaining that they can't even get the back button right for the store.
It's things like that which separate a tech toy from a console.
Working with game studios is one thing, having massive widespread appeal so they feel compelled to take you up on it is another.
As for the "homework" you mentioned valve should've done, they did it. They've mentioned that they'll work with game studios of all sizes to help them make their games compatible on launch with the steam deck, both in technical terms (proton on Linux) and in usability (ui on a small screen).