Blockbusters will still happen, franchises will still bloom, but their impact craters fill in much more rapidly with new materia.
I think your looking at the question through the prism of being a sci-fi fan rather than what is universally popular. Wars is definately one of the most popular franchises of all time - but Potter not only hit box office records but is one of the best selling works of fiction ever.
I think SW is without a doubt in terms of sci-fi one of the biggest impacts to the genre. However, Potter accross all age groups and that its been successful both on screen and in books eclipses Wars in terms of its cultural impact.
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.
I don't know whether they'll retain the massive cultural impact in 20-30 years time, in the way Star Wars and (to a slightly lesser extent) Star Trek has. Look at the Matrix: everyone and their uncle was quoting from that back in the day, and now no-one really talks about it any more. That's what I mean when I wonder if the age of the culturally-massive franchise is over.
I would agree that Potter currently seems to be the closest to SW in recent times, though. But we'll need another 20-30 years to be sure.
The poster above me rightly says:
. I think people have started to forget exactly how massive Harry Potter fever was.
The thing is, I'm not sure people have started to forgot how massive Star Wars still is. It's the gold standard by which media franchise success/impact is measured. Plus, Lucasfilm is very careful about nurturing and protecting the franchise. It'll be interesting to see how Rowling, her publishers, and WB manage Potter over the decades to come.