Don't get me wrong I love my Steam Deck... But with its mass-market appeal the Switch 2 is going to steamroll over the handheld gaming device market.
As streamlined as it is, the Steam Deck is sometimes finnicky due to it being a PC (which is an amazing thing for tinker-minded folks). Plus it cannot play some of the most popular PC games on the planet (Fortnite, LoL, Roblox, etc).
Windows handhelds are even more niche, because dealing with Windows on handhelds... Man that takes some serious dedication.
I would wager that Nintendo is going to sell at least 20x more units as the whole handheld gaming PCs lineup combined.
Most of the time it's because client-side anti-cheat is not being well or at all supported on Linux.
For instance Roblox used to run on Linux through launchers such as Grapejuice or Vinegar. However the devs now explicitely block Roblox player on Linux due to a large quantity of cheaters coming from there according to their stats. Since their actual Linux userbase is minuscule compared to other platforms, they just chose go the easy (and probably financially sane) route.
As for Fortnite, it's all about the ROI baby !