I understand your point - he'd go back to Real Madrid a better player if we developed him. (I believe that's your point, anyway? Please feel free to correct me.).
BUT - what if he develops with us, and then he loves Manchester and wants to stay? What if he's superior to what we have right now, would improve us next season and then we get into the Champions League? He leaves, but he leaves us in a better position than when he started? I know short-term thinking isn't ideal. particularly as we've had 7 years in a row of short-termism. But surely we must think about the coming season as well? We can't simply continue to write off our upcoming seasons, as we did last year. We need quality, regardless of age and country of origin, and we need it right now.