A hypothesis: Religion goes to the Megachurch to die. The Megachurch will continue to grow and flourish for the next generation or so, but then it will be abandoned. As the Megachurch is a Protestant church whose main talent seems to be the decimation of of Confessional Churches (Catholic and Protestant), the abandonment of Megachurch will constitute at least the end of Protestantism. Catholicism is too historically unpredicatable to predict.
Reason: Religion consists in things that bind us. The soul of the Megachuch is this: lack of anything that binds. Outbreaks of emotion are encouraged, even set up as ideals. Community ties are rendered impossible through becoming a faceless man in a crowd. The atmosphere is entertaining and liesurely, and nothing is less binding than liesure. No morality, whether progressive or traditional, is mentioned. The distinctive doctrines of Protestantism are essentially forgotten, or have softened into platitudes. So too the old antinomy to Catholicism. All beliefs concerning the Trinity or the Incarnation would immediately fall under the slighest popular attack.
Reason for the decine: Hatred of man. Christianity is incompatible with a hatred of human existence: God became man, God even is a man. Modern culture believes strongly that human existence only derives goodness from something else: and so we poison ourselves into infertility, we choose to neuter ourselves like dogs, we let anyone abandon his offspring for absolutely no reason, and it is unthinkable to suggest otherwise.
This situation has, of course, existed before: if Polycarp or Iranaeus or Justin Marytr or Agatha or any of the Apostles were to take stock of our situation, they would think that it was comparitively good: remember that the world of the early Church was one where baby girls were usually abandoned in the woods to be picked up and raised as prostitutes, where people watched other men kill each other for sport, where homosexuality was the rule for married men, and where divorce was an unquestioned right for both sexes. But I see nothing in the megachurch that can stop this sort of thing. The megachurch seems to be more a participation in this sort of thing.