[Do people still talk about Potter though? I'm asking genuinely; I'm not in the demographic to know. What I mean is Potter was clearly truly huge, that much is obvious even to someone like me outside it. But how much of that was reliant on a constant refreshing of enthusiasm through new books and movies? Now that those have died down, will it be sustained over 20-30 years? I've no doubt the books will continue to be popular but once the buzz of new material fades further, they'll have to compete with all the pre-existing children's classic franchises: Narnia, LOTR, etc, etc.
Over the holidays I visited Universal Studio's Orlando Florida theme park and the Potter portion of the park was the most crowded by 10 fold.
It doesn't get as much talk as the hype up to last summer's movie but it's still there.
If Rowling is smart she'll relinquish some of her control over the franchise and allow others to write prequels and sequels.
I'm betting though that Rowling won't easily give away her pen to someone else though on the franchise just as Lucas was reluctant to do.
One thing you have to give Trek credit for is encouraging fans to write fiction and even create their own fan shows which helped keep the love affair with the franchise alive.