A lot of people seem to have an inflated view of what the mentor system is. It's existence is to fill spots in rare duties that aren't popular anymore and to keep the duty finder moving, period. SE understands this and that's why they put a long mount grind to go with it. That's how it fulfills it's purpose. People leave various duties for a variety of reasons all the time regardless of being a mentor or not. I would imagine everyone here has been in a duty that someone left at some point. The only reason you're feeling like it's only mentors leaving these ex trials is because without the mentor system these duties probably wouldn't even pop in the first place most of the time.