Look around, it's a damn fad. There is people collecting toys, watching the show every day, making fan art, converting others, dressing up like a pony, etc. You know what this whole thing reminds me of? This whole Pony fad reminds me of pokemon in 1998, this fad is so loving similar it's god damn creepy.
Alight, let's take a look at some other popular TV shows, shall we? For instance, I watch Doctor Who. I was "converted" by my friend into watching it and I happen to own some merchandise. And I've also seen a lot of fan art of it and people dressing up as the doctor for conventions and such. It is liked by all ages, kids, teen, adults. This fits perfectly into your definition of a "Fad". But this show has been going on since the 50s and hasn't lost popularity.
Now your definition of a fad can fit into every TV show in existence, even if it's a kids show or not. Pokemon, Yu-Gi-oh, Family Guy, Simpsons, Lost, Star Trek (A big one), Transformers, Battlestar Galactica, etc. You get the point. All of them. They have all become "fads" according to you, making your argument of people obsessing over this show irrelevent. And if you like any other tv show, you're now just being hypocritical.
I dont even have to be a brony to tell you you're arguing to provoke and not to prove any valid point. I'm saying this from a neutral side. Just stop.