You'd be surprised, a lot of kids opt not to go the legacy route simply because that's where mom/dad went and they want to branch out. I'd say getting admitted has little correlation to success in school, anyway. Plenty of people fail out. To your last point, it would be nice if legacy/donor admits didn't cut into the number of offers extended to everyone else.
this is actually pretty rare at very selective institutions. Harvard has a graduation rate of about 96%. most elite schools tend to be in the mid to high nineties.
