The Steam Deck is a handheld gaming platform entering a handheld game market dominated by the Nintendo Switch for real games and phones/tablets for useless distractions. People playing on phones and tablets aren’t the same as people playing on the Switch, and the Deck will compete much more directly with Nintendo‘s handheld.
Can the Deck dethrone Nintendo as the king of real mobile games? Ever since the OG Game Boy days, Nintendo has had the lion’s share of mobile gaming due to excellent experience and quality. All of their consoles have delivered a solid platform with a good core lineup of software and a nice breadth of more niche titles to appeal to different folks. Additionally, Nintendo has always targeted a lower budget gamer with its handheld systems, making it easier to sell lots of systems.
So how does the Deck stack up to that? Hardware wise, it seems to be essentially a (powerful) netbook in the form factor of a game console. Thanks to some software wizardry (that I’ll get into in another post), the Deck will be able to run most games on the Steam store out of the box. I’m 96% sure it will “just work” for most games, just like the advertisement promises. I’ve been following the tricks that Steam is using for some time, and it appears to all be pretty solid. All of this means that the system will be a solid platform for running games on.
With the entire Steam library available, the Deck won’t have the problem that many consoles have on release: limited software. On the flip side, they won’t have the benefit of software exclusives either, and this is where I think the Deck will never overtake Nintendo’s secret sauce. The real reason you buy Nintendo is to play Zelda, Mario, Metroid, you name whatever your favorite Nintendo franchise is, that is the reason you buy a Nintendo console.

Regardless, I honestly don’t think they’ll directly compete with each other. The Deck is essentially a gaming PC, and is priced like one (it will even have a Linux desktop if you go looking for it). You won’t get the console experience in the same way you would from a Nintendo console, and so I think both systems will coexist the same as console and PCs have for decades now.