Frankly, I don't think there is a way to predict which applicants are going to become researchers. If they have an interest in research, experience doing it, and a clear vision of what an MD/PhD does, how are you to say they aren't going to go do it?
I think MD/PhDs who fall off the wagon have experiences that lead them to do so. Maybe they have family and kids and they can't justify to them that they're making peanuts when they could be out there making 200k+. Maybe they had a horrible lab experience due to a bad mentor, getting scooped, or just because their experiments never did work, and they've had enough of research. Who knows?
Besides, do you really expect pre-meds to know really what MD/PhD is going to be like for them? Sure, they can have MD/PhD role models, but how do they know what they're going to think in training and in the real world? Maybe they just won't like it? Some people propose that we should go back to forcing MD/PhDs to do some amount of research.
Maybe there are some amount of pre-meds who really are just ignorant and don't know what they're getting themselves into. I've heard stories from physician-scientists about these types when they're speaking of themselves. Then again, I've seen people who have always known the MD/PhD life (maybe they even had parents who are MD/PhDs) and who have always wanted it, who had to decide it wasn't for them for one reason or another. I see both types (clued and unclued) in my program. Is it possible to predict which will become the researchers? I don't think so.