Being more general and less snarky - I think that, probably, you (personally, as a single hiring manager) can get away with "only hire CS grads from great universities with firsts", if you work for somewhere easy to hire into (Facebook/Google/Twitter/whatever), because they don't find it hard to find people, just to find ones who're good enough. If (as I suspect by your estimation of the costs of mid-hiring) you're working for a no-name start-up, you're not only cutting out the brilliant people without degrees (and I've worked with many over my career), but you're also cutting out the excellent graduates who won't want to work for you because you're hiring a monoculture.