There are plenty of supposed Steam Deck killers available in the portable gaming space, from the Asus ROG Ally to the Lenovo Legion Go and all the Ayaneo and ONEXPLAYER devices in between. But none of them have managed to outright slay the dragon that is the mighty Steam Deck yet.
I'm honestly surprised Valve hasn't made an open source de-Valved SteamOS similar to what Google does with Chromium for these other devices. Valve likely isn't making much from hardware sales alone, with most of the value for them coming from the Steam store being a mobile gaming storefront as well as moving users away from Windows where Microsoft is looking to compete with them. Getting competitors to run Linux distros with a user interface designed for mobile consoles would boost the amount of Linux gamers which would make Valve less dependent on their competitor Microsoft, make developers more keen to support Linux, and spurn further development for Linux gaming tools. These other manufacturers will without a doubt support Steam as a storefront since Steam is such a dominant force in the PC gaming market so users of these other devices would still be in Valve's ecosystem