Atari Lynx, Gamegear, PSP, Vita didn't hurt Nintendo and all of them were far superior when it comes to hardware, compared to Nintendo's counterparts. Besides the Switch is over 4 years old at this point. Even if the Steam Deck is wildly successful, it might be just enough for Nintendo to release a successor to Switch earlier or do the price cut of their current consoles. Either way, the consumer wins.
Moreover, they didn't hurt them despite being there to directly compete with them, which is not the case of the valve handheld.