I think endurance bikes came first, but they were fairly contemporaneous with gravel bikes. And endurance bikes have come to look a lot more like gravel bikes.
I think the people that say gravel bikes are a fad just don't recognize the source of their popularity. People want these bikes, they see all sorts of uses for them other than gravel. What they don't want is a tight clearance racing bike that would fit in just fine at the Tour de France*. It used to be that if you wanted a higher performance bike, that's what you had to get.
*if you want this, it's available so don't fret.