That is NOT what it all boils down to, though, is it? The model developed by Chelsea and others, shows the way. The ideal goal is to get one or two players per season to your own first team squad, but developing others to the point that they are saleable to Championship / League One clubs, as a funding source, to help the academy pay for itself.
Chelsea just like every other club also release players. I'm sure the club would have preferred to sell Cashman rather than letting him go, but if they find it unlikely that anyone would pay for him its not really much to choose from.