I disagree highly aspen. Anyone who looks at the Harry Potter "situation" would be very wrong to declare it a fad. It's been around for (if I'm not mistaken), around 5 years already...and another 4-5 years of popularity are guaranteed, as we likely won't see the 7th and final book (which will undoubtedly receive much fanfare...as it will tie up basically all loose ends) until at least 3-4 years from now. Toss the movies into the mix and there's no way you can expect it to be dead in 2. All in all, that'll have been anywhere from 8-10 years. 8-10 years is a fad? No way.
Children grow up...and new children are born. And children grow up enough to the point where they can read. My younger sister Grace has read all four books; and the next-youngest child, Addie, is quite likely to read them soon herself. I have little doubt at least one of the next three children will read them at some point, too.
The actors growing up is not a big issue; they are filming them very quickly. The worst that happens is they replace some of them. You think the third (or fourth) will go straight to video? I'll take that bet...just tell me where to sign.
LOTR has sold something like 100 million copies worldwide in over 50 years, unless my memory is faulty. The Harry Potter books have sold basically the same amount in around one-tenth of the time it took LOTR to hit that mark.
Ironically, I agree with most of the other things you've said: the second LOTR movie may make less than the first (that's my opinion...the story of the second, to me, has always been less interesting, but I can't speak for the masses here)....but it will undoubtedly open with more, barring some unforeseen major event before its release.