I'm an alumni of Usenet and IRC from back in the early-90's. I can assure you that there's always been (and probably always will be) demand for anonymous forums, into which trolls can project sexual frustration and feelings of powerlessness.
I was a bit too old to know about Yik Yak during its heyday, but I can only imagine the troll-appeal of anonymity PLUS being able to target local real-world individuals by name. Old school trolling was limited to targets who were non-local, and by and large anonymous themselves.
So I have no doubt about the market demand from trolls. I just don't understand why there was ever any demand from ANYONE ELSE. And I certainly don't see a path to monetization.