"What advantage is gained by moving on the food chain from predator to prey?"
Too many of that type of predator hunting too few prey animals.
This was not a giant dinasaur. They probably ate something the size of rabbits.
Too few rabbits. Lots of grass (or whatever).
So adaptation rids them of their choppers in favor of all grinders? You buying that? I'm not. The omnivore is much better suited to survival than the herbivore or the carnivore. Sounds like they were selected out.
If you told me the creature had a mixture of grinders and choppers I'd be more inclined to think that it was transitioning from something to something because I could understand an advantage. As is, I don't see it. But then again, neither did anybody else.