I agree completely with Njee
My lbs which used to sell the odd road bike now has only about four mtbs. One very cheap, a Saracen Ariel they’ve been trying to sell for two years (yes really) and two others. The rest is cheap hybrid stuff or road bikes.
15 years ago they’d have 50 ish mtbs easily and they were flying out the door. They said that they never get any enquiries about them these days.
Back to the op point, as mentioned above 650b not different enough to bin what works, but due to lack of choice most people will move across as and when they replace either frame or complete bike.