Well, I would agree that demography plays a role. But I would suggest Christianity (all branches) in Europe and America is losing members-not gaining. 85% of American call themselves "Christians". Do we really believe that 85% of Americans are Christians?
Were they ever other than the mythical America of yesteryear that only existed in the minds of evangelical Christians?
Remember the Half-Way Covenant? It provided that children and grand children of church members were partial members of the church who had to accept the creed and rules of the church but did not have to follow all the teachings of the church nor could the half-members receive Communion or vote in church polity decisions.
These problems have always existed. It's only due to the myopia of nostalgia that we think otherwise.
Sadly, I do agree with your conclusion. I believe that the American Catholic Church of the future will be, precisely as Pope Benedict predicted the European one would be, "smaller and leaner." I definitely see a rebound in the fervor of the core of Catholic, American youth, amdist a continued weakening overall... a rebound in the overall picture could well be brewing, however.