Well that's true, but I think that if the youth academy really is a priority for the club, with good scouting, better facilities, giving youthplayers a chance, it's possible. Lvg always had the youth academy as a priority, we all know how many great youth players have come from it since.
Ajax for example just started a new revolution through Cruyff, the youth had to be a bigger priority. Since then, many great talents have come through the ranks, Alderweireld, Verthongen, Klaassen, Eriksen, Kishna, El Ghazi, Fischer for example some new brilliant talents are coming too. And ofcourse Ajax isnt as big as United and their players dont have to be of such quality, but if I watch their youth teams in tournaments like Aegon, or the Youth League, they're arguably the best and produce some great talents. So I think theres a bit more to do than just luck.