Ah, but I'm not saying that Ivy/Oxbridge grads make better employees all else being equal (in particular salary). I'm specifically saying that on a financial/job hunting basis, it makes more sense to pick a prestigious university and "useless" degree than a "useful" degree at a non-prestigious university, in aggregate.

When you are on the other side, other variables come into play. For example, elite grads might be generally overvalued and for the same money you can get a much better community college drop out (who is on the tail end of his own talent distribution), because the market undervalues his skills. This is something that comes up a lot in the Valley and the last team I've personally hired was about half no-degree and half not-famous-degree, so you can guess my position (we hired based on technical capability and more or less ignored CVs).

Whether or not I agree with you on Goldman and McKinsey's intrinsic value adding capability, I have observed that people doing a stint at either are basically guaranteed nice employment for life, fast track to the best management jobs, get a premium on hiring, and so on. Which is what the OP was concerned with.