Every fandom pretty much goes through a period in which its popularity explodes all over the internet. The My Little Pony fandom isn't dying. It's just settling down and becoming more tame.
Sure, the ratings are low now, but a lot of people watch the show through streams so the numbers aren't all that accurate. For all we know, people are starting to watch episodes online rather than on TV.
But I do want to compare MLP to another big fandom: Harry Potter.
Once upon a time, the Harry Potter fandom was insanely huge. While the movies and books were still coming out, it had a very, very large fanbase. But when the series came to an end, it looked like the fandom was dying. Not exactly. If you go on tumblr, you'll see the Potterhead fandom is alive and well. It just seems small because there's not as much new canon content being released as there used to.
So really, the Bronies aren't exactly dying out. They're just settling down. Their peak is over and now they're more level with other fandoms.