It won't even hurt Switch OLED. Not by much at least. The thing about Nintendo consoles and even upgrades is they are purchased by people who want Nintendo games. The upgrades might be new users, or they could be users upgrading. Either way, it isn't a market that the Steam Deck is part of. Yes it has a few playstation games and a lot of Xbox games. But no one has actually come close to Nintendo in the portable market, the closest one was the PSP, which was still outsold almost two to one.
I think this is a great product. Even the Vita). But no one put a dent in Nintendo's marketshare. This won't either.