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.
Nintendo are in a league of their own when it comes to Handhelds, Normal Switch will outsell Steam Deck 2 reasons
Most parent's wont fork 5,000 dollars scalper price for Steam Deck , plus this morning I saw another Switch V2 on Walmart and someone bought it , demand for Switch is still very strong even today 2021.
Cute and honest Sega Saturn fan, also noone should buy Sega grrrr, Sega for life.