It reminds me of the Star Wars Movies.....
I remember seeing Star Wars as a kid...saw it over and over again...then couldn't miss Return of the Jedi. Then had enough...didn't even care what happened in the next "episode".
We have the newest one (hmmm...maybe called Phantom Menace) on VHS and I've never watched it. (DH bought it).
So I suspect it will be the same with many thousands of viewers...they will just lose interest. But didn't they say (the book PR people) that over the years some of their readers haven't continued on with the series but others became fans -- so even though you may lose x amount of viewers with the movie, you will gain x amount as well.
My niece is 11 yrs old and she zipped through books 1 & 2 like there was no tomorrow...but she hasn't been able to zip through book 3 at all. She claims to still be a HP fan and has books 4 & 5 but I don't anticipate her ever reading them as she seems to have lost interest in the entire thing.
Okay, I'm going off topic here...let me steer back on course...
I think yes, they will lose lots of viewers for one reason or another...but will gain others. I think the movies will continue to do well at the box office but may not break huge records.
As much as I "like" (don't want to use a stronger adjective here) the movies, I hope they someday make a more grown up version...you know what I mean...the movies are so shallow in comparison to the depth of the books. The LOTR's movies seem to be geared to a more grown up audience while the HP movies are geared towards a younger audience.
I suppose it's much too soon to be thinking of movie remakes...that'll be some 25+ years in the future...and I'll be old and gray by then!
Those are my opinions...for what it's worth