You're right.
Our Academy has been very productive though, especially for lower league clubs. With a couple of minutes of research players who left our clubs playing in:
Premier League: Simpson (Newcastle), Bardsley, O'Shea, Brown, Richardson, Campbell (Sunderland), Morrison (West Ham, remains to be seen if he even plays), Gibson (Everton), Fox (Norwich)
Championship: Norwood (Huddersfield), Drinkwater, James (Leicester), C. Evans, Chester, Stewart, McShane, Dudgeon (Hull), Ajose (Peterborough), Cathcart (Blackpool), Martin (Ipswich), Eagles (Bolton)
League One: Devlin (Brentford), Gray (Stevenage)
Abroad: Pique (Barca), Rossi (Villareal), Pogba (most likely Juventus)
Most (not all) of those only left over the last five years. It's clear to see most of the time there is only one way after leaving United: down. Not to say those players aren't having decent careers mind...