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.
IIRC, Valve has said in the past that they’re willing to support other devices. In fact, GPD had said that they were in contact with Valve to bring support to their devices.