Well, fads always change in the bike world. Gotta have a new shiny object.
You are correct though. In the last few years, the industry touted having wide 650b or narrower 700c rims, so you could have two sets of rims for different conditions. I think they figured out that 99% of the people just put one set of rims on and left it at that. 700c has better rollover characteristics, so these days they are just making bikes with clearance for wide 700c tires. Kinda removes the need for 650b. The big exception of course is if you have a small(ish) frame. To get the same geometry you'll need smaller rims at some point. 650 is great if you can't be riding 29'ers on your gravel rig.