Tbf I don't think you can really blame Fergie that none of his players turned out to be world-class managers. Those type of people are pretty rare in football and you need a certain element of luck. Even the football culture in the country plays a part in it. For every Zidane, Conte, Deschamps, Guardiola, there's Inzaghi, Gattuso, Seedorf, Henry, Adams, Souness, Shearer. Even like Bobby Charlton flopped in his first managerial stint and got the club relegated.
When's the last a good or potentially world-class manager came from Britian? Nevermind someone that actually win titles.