I think it was
@jdog that pointed out that the problem with 27.5+ was that nobody put tires with actual tread on to save weight and that killed the benefits.
I definitely would say Fat Bikes were the most over-hyped trend. Not saying they don't have their uses but that was heavily pushed as the big new thing for a short time period until almost everyone realized they suck except in certain cases and aren't great for a multitude of trail types.
Gravel bikes were over hyped but their usage case is way higher and really most casual road riders would be fine on a gravel bike just as most road riders shouldn't have been buying the racier versions of road bikes. Heck I sold my road bike and use my cross bike as my road/gravel bike.
I'd agree with
@UtahJoe about singlespeeds but it was quite a small minority hyping singlespeeds. IE, those that already owned them.
E-Bikes are over hyped as it's the "new thing" out there for shops to push, but they aren't going to die out like fat bikes and singlespeeds.
I've lived down in WNC for 13 months now and the closest I've seen to a fat bike on the trails here is I saw a guy on one in the parking lot with his kids at Dupont. Apparently nobody fell for the fat bike hype here.