Before I go, I'll just make one more point:
There have been many fads. Furries, Housepets, fsjal... There were also megathreads created for such. But why did they go downhill? Because some people chose to start arguments over it. This pretty much would not have happened if you were tolerant of a few pictures being posted in different topics.
If people hadn't chosen to start posting things intended to piss off a group of avatar fad users, none of this would have happened. The Bronies are ultimately not to fault.