I don't necessarily disagree, and I think the WC fact is a good one to remember, but at the same time there are plenty of examples from this season alone worth considering. Claudio Ranieri and Quique Sánchez Flores have both been very impressive in their EPL managerial return (after 11 years) and debut, respectively. And perhaps to a lesser extent, Slaven Bilic has also been solid (albeit inconsistent) in his own EPL debut.
At the end of the day, I think it all comes down to how committed the coach is to buying into the premier league and developing a mixture of talent. The best example of this is probably Pochettino, who Nevok and I were talking about above. In his first BPL seasons, coming from Espanyol, he immediately showed a strong interest in strengthening the youth program, which is something he's done excellently with Tottenham as well. Mixing in some high quality foreign talent as well (guys like Lloris, Alderweireld and Eriksen), he is nonetheless building the foundation of his team on the backs of young, committed, British players like Kane, Dier, Alli, Mason, Pritchard, etc. In effect, I think he's done a heck of a lot more for developing British soccer these past couple years than guys like Alan Pardrew and Harry Redknapp and so on and so forth. He's struck a good balance there, it's paying off in a big way for him at the moment (though, difficult to see how much longer that will last), and as far as I can tell he is no less capable of doing so as a foreigner.