While other vendors continually push out new handheld pc models, sticking similar internals into different shell designs and gradually bumping up RAM or the Processor, the Steam Deck just keeps selling like hot tasty cakes.
The big thing is that the Deck uses Linux which allows Steam to provide an amazing interface
For most people, i think it's just that it has a good UI. They don't bother whether it's linux-powered.
Maybe it's possible to create a good UI in windows as well, but it will be bogged down by 100s of unwanted processes.
Absolutely. But they want everyone to know they are using Windows, which if there was a custom shell then they wouldn't.
They have >95% of the market, they don't care about making a good interface for TVs as they already lost it to Android, and they don't care about making a good gaming interface as they already have XBox.
Windows users are an inconvenience, that they want to milk.
What's even more funny is that they already have two, somewhat decent, interfaces which are meant for touch; Win8 tablet UI and Windows Phone Live Tiles UI.
If they want, they can easily work upon them, make them controller-friendly, and bake it into Win11. But they are too stubborn to do it.
They've also got the Xbox interface for controllers, the reason they don't make Windows better for this scenario is that they already have XBox and making a SwitchBox interferes with their plan to make everything shit.
I can't speak for Epic Launcher games (I know that Heroic Games Launcher exists but I've not personally tried it with Epic games) however Blizzard games absolutely can be played in SteamOS - you can utilize something like Bottles or Lutris to install the Blizzard launcher, and then download the games from it as normal and run them. It is how I originally played Diablo 4 on my Deck before I picked it up again on Steam. I swear I remember both Bottles and Lutris even having an "Add to Steam" option to integrate shortcuts directly into Steam (and thus, coming up in the Gaming Mode UI) but don't quote me on that one.
Blizzard games are actually some of the earliest non-Linux-native games that I remember running very well back in the days where we just had Wine (before Proton, DXVK, etc) which is something that always impressed me.
The number of games that won't start on deck because it uses Linux is incredibly small. If a game won't run well it has little to do with the deck using Linux instead of Windows.
This is just my experience, but I have had next to zero issues running games on the Deck that were related to the platform. Most problems I've encountered are along the lines of the game being KBM-centric and it being difficult to play with the controller inputs.
The only Linux-specific issues I can't think of are related to trying to install or mod games outside of Steam (Skyrim in particular is far more difficult to mod on Linux than I expected).
There are easy ways to install and integrate epic games for play into the steam UI (on steam deck). I am playing remnant from ashes multiplayer on it everyday.
All fair. I haven't tried either with the Deck, though that's more because I don't want to try games from either platform with a controller. I have had success running both on my Linux desktop, though.