Depends on your definition of success.
Will it sell 150 million units like Switch is going to? Certainly not. It'll likely do 5 million, maybe 10 if it really hits it off. It's a massive cruise ship more than twice as heavy and 2-3X more expensive than a Switch Lite, with less battery life, can't fit in your back pocket, and doesn't have any exclusive must have games that will draw ppl like on Switch.
That said, 5-10m for a variant form factor of an already existing platform is plenty good enough. It's got a massive library out the gate (even if some games won't run via Proton or, if they do, they may have quirks and niggles even with Gold rated compatibility). 80% of my Steam library, even now, is Native, Platinum or Gold, offers Gyro and Steam controller config options for every game.
I think this device appeals to 3 groups: Steam players who want portability. Lapsed Steam players who see it as an easy re-entry point. And Nintendo Switch gamers who have a deep respect and appreciation for the portable gaming experience and will see this as the perfect complement to run those heavier titles Switch doesn't do too well with.
And I don't think it's going to cause some massive rift between Switch and PC fans; on the contrary, it's going to unite them. I reserved a 512 GB Steam Deck, as well as a Switch OLED, because I love portable gaming. I think the biggest boon the Deck has in its favor is Nintendo Switch gamers who will evangelize its merits.
If they manage to release a slim version- change to 16:9 720p and remove bezels, shrink the screen to 6" and redesign as much as possible... if they could manage a slim version around 3" shorter and 200 grams lighter, with size and weight comparable to Switch (forget Switch Lite, that's never happening) I think Steam Deck could blow up.