This is true, to a degree. If you consider the mid 80's- late 90's there were a lot of americans in the top 100 consistently year after year. And I'm sure the case was the same back then as it is now, there are a lot of other sports which takes priority over tennis. So there are other factors involved, Australia has this dilemma as well, I think the surfaces now are more conducive as a whole to players from south america etc. Even when they replaced rebound ace to plexicusion it isn't for the benefit of the players that australia has been producing, maybe Tomic now but in general it hasn't and the rankings show this.