The Steam deck won't "kill" it but it will hurt sales. People got the message "this is what is possible" vs. the Switch OLED being based on 7 year old 2014 cellphone hardware (the Deck is using current laptop hardware, not mobile hardware, it is much more powerful for both those 2 reasons).
I think the only issue for Valve is the device is a little big and heavy. Engineering isn't as good as Nintendo's OLED model even if the APU is 10x better. Nintendo just needs to get their Switch 2 out soon and it will match the Deck in performance.
Apple badly needs to enter this space with their M1X product. $600 for a gaming device it would sell crazy with an M1X.
Don't pretend the Steam Deck isn't popular. It has now been pre-ordered to Q3 next year. In a week if you place an order you might have to wait until next Christmas 2022.
Bold: As if this has any meaning.