We have to disagree on your first statement. A league(s) is/are essential to competing at the World Cup because it creates a sustainable player pool to source from year after year. We cannot depend on foreign clubs to develop our players because by in large they won't play for us. If that player production comes from a semi-pro league (ala NZ) or a professional one we still need consistent player production.
Julian Deguzman said playing for Canada almost ruined his european club career because its a harder sell for agents and clubs hate low ranking games and pointless games due fatigue and injury - plus travel. It also helps to soften the blow when players don't sign up for Canada because we have more depth.
I agree, the handful of players your refer to would be enough but what about in 4 years time? Where is the next round of players coming from?