>I've personally seen a large number of people with at least one degree fall apart under the real pressure
There are of course false positive/negatives with every heuristic, I don't think that makes them worthless.
> people who go to college at all are more likely to be from well off families and have educated parents.
The studies on how a diploma effects future earnings obviously controlled for that, as well as many other things. You can go ahead and check the studies, they're pretty thorough. College is a great investment for many, even if it is much easier for the wealthy to obtain.
It might not be a good signal of "can overcome adversity" because everyone has one, but not having a degree could be a good signal for "can't overcome adversity".
And I would be remiss of I didn't point out that people who go to college at all are more likely to be from well off families and have educated parents. It's a "great investment" for those who already have a huge head start.