Whatever she writes next, Rowling is sure of one thing: It won't be as successful as Harry Potter. "I don't think I'm ever going to have anything like Harry again. You just get one like Harry."
True enough. I've seen a number of authors get caught up in the externals -- money, sales, contracts -- and lose sight of the real motivation: to enjoy the writing. When you do that, you harness your writing to something other than you. Technically, this is called extrinsic rather than intrinsic motivation, which works better. I think it is very reasonable that JKR isn't expecting to do this twice.
I suspect that this is one of the reasons why Stephen King "retired" from publishing -- not from writing, you note, but publishing. People were saying he was losing it because his sales went from 2.5 million to 2.3 million in successive books. Good lord! The vast majority of writers would be lucky to have one-tenth of those sales! And a lot, even one-one-hundredth! (Fact: typical paperback royalties are about 5-10% of cover price. So you get maybe fifty cents for a book if you made a good deal.)
Ultimately, it can be very freeing -- you have no expectations but your own: to have some fun with your writing! which is why most people get into it in the first place.