Well in stable market conditions, the tail of the group of people who get diplomas, ranged by their ROI, will get somewhat negative overall ROI. That's how it should be - i.e. most people - the smarter ones for who education makes sense, get positive ROI, and some relatively small group of people who can't really benefit from education but have fooled themselves into thinking they can, get zero or slightly negative ones. Then comes a group of people who clearly can't benefit and know it. The system reached a stable state - colleges get maximum number of clients they possibly can and get max revenue, it doesn't make sense for many more people to join in, and it doesn't make sense for many existing to stay if prices go further up.
As with oil production for example (see 'marginal barrel') people at the end of this tail don't really benefit, but the system in general is still good, most people benefit.