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.
Is killing the point? Just make it all better. Get rid of DRM and subscriptions to play your own games, let people use LAN and their own servers for multiplayer.