29ers have been adopted by the majority of manufacturers and pushed in lots of applications. The majority still offer a long travel 29er variant where supposedly 650b would ‘excel’. I can only think of Giant that scrapped 29er.
The 650b segment has replaced the 26” bikes and a few 29ers. The 29er size is stand out and adopted mainly by the commuters, leisure cyclists and XC, trail riders, as well as the giants.
29ers are here to stay. You won’t have any issues sourcing parts for them for the foreseeable future.
For what it’s worth, I own 3 29ers and love them. I think i’ll only go 650b for long travel or park/pump track bikes.