I think the main reason for leaving is that the kids have introduced their parents to their friends not in the org and the parents wake up after seeing the world isn't bad after all.
That could be a reason. That would never fly in my family, though. I think that elders are placed in a bad position. They are placed in positions of authority, which some may find cool, but time after time becomes clear to many that they just don't have the life experience, leadership skills or political spine to continuously deal with people's personal problems and provide the very bad advice that they are bound to give. And then, they don't even get a thank you from their peers nor (horrors!) the WT.
Many elders are just burned out. Just my opinion.
Have you seen whole families leave? If so, for what reasons?
Not all at once, like a family Exodus. However, I've seen family members following the steps of some who leave, and the the entire family being out, and yes, like you say, happier, healthier and more vibrant.