This is pure opinion but… If there was a 29er boxxer or 40, and a selection of 29er dh rubber, and some big hitter teams with a desire to win on a 29er dh bike, I reckon world cups would have been won on 29. But it’s always meant second rate kit being played with by smaller companies. Even then you get your occasional mitch gruffalo, or gwin at sea otter. Or Graves on his enduro bike at champs.
I’m not sure! The Manitou Doraro takes 29er wheels and they are regarded as good forks. There’s plenty of suitable 29er DH rubber available from Maxxis and Schwalbe (etc.) and some of the big guys have/had suitable frames- Foes, specialized, Intense & Niner (?).
I supposed that if everyone wash pushing 29er for DH (and winning) too then 26”/650b would be obsolete as 29” would cover all riding disciplines (from a marketing/racing perspective).