It's because they've lost touch and they don't know what their fans want anymore. The only one so far that did it right was Invader Zim. It didn't miss a beat, and felt like an even longer episode of Zim. (Which is good) They just did everything right, and it was actually great, and something I will rewatch.
But the new PPG? Teen Titans Go? The Rocko thing? No. Just no. They're not good, at least, not made for the old fans, which is why most don't like it, and I can agree with that. But it's still pretty bad no matter how you look at it.
I doubt the same will happen to FiM. As much as I want them to keep going. I don't think they should ruin more than they already have. The ending is fine, rushed sure. But it's fine, it did what it needed to do, despite now showing what we'd actually like to see, and- the rest would be spoilers for who hasn't seen it yet.