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.
Worth noting that in the UK until recently cost and university were un-related. Indeed, it's one of the aspects of the modern system that infuriates me. I despise the idea of a bright child choosing a mediocre university to save 2-3k of student debt per annum. I can't believe it's good for the state of society.
It's also worth noting that the terms of student debt in the UK are far far less usurious than in the US.
I'll say it: You just hired for cheap.
The reality is - more nuanced. Certainly there are many who consider both GS and McKinsey to be very bad things indeed, and not in any way a professional niche that high quality graduates should aspire to.
Perhaps there may be more to "intrinsic quality" than an obsessive work ethic and social connections?
My experience has been the opposite to yours. People who don't have technical degrees but have been trained to consider themselves experts in business management generally overestimate their own skills, and vastly underestimate the skills and insights of those (nominally) "under" them.