Success is relative. The Steam Deck is only unsuccessful if you consider the goal of the device to be "outsell Nintendo". I would argue 4 million units is not merely a success, but a massive success.
The Steam Deck also had no marketing and is not sold in retail stores. It's also been a success in kicking off a whole product category of handheld PCs, of which most games will be bought on Steam.
That's also debatable. Switch 2 sold 10m units in 6 months compared to the Steam Deck's 4 million in 3 years ¯\_(ツ)_/¯
The Steam Deck is niche even among the gaming crowd.
>COD, Fortnite, LoL players can stay on windows. I’m happy to play newest indie game on my Linux machine
This is the mindset that makes the Steam Machine DOA if not priced correctly. No one will pay $800 just to play Hollow Knight in 4k