I hate to say it, but if MS released a competitor, it will probably outsell the Deck 5:1 regardless of quality, if only because of the advertising reach. Your average non-gamer has never heard of Steam. Everyone and their grandmother know MS and would therefore be more willing to get one for their kid.
Edit: I suppose I should explain a bit. People here are comparing Steam DAUs to console DAUs. That’s not the same as sales.
All of those users are already playing on a computer. Also, many of the most popular games on Steam I are free and low-spec. A lot of Steam users are not spenders.
Compare the 3.7 million Deck sales to the 2.2 million Switch 2 preorders (and the 150 million Switch 1 sales) before it even hit shelves. You can’t even buy a Deck in a store and you won’t see an ad for one. If MS makes a handheld, they’ll have billions at their disposal in advertising.