Stephen King's IT: What The Ending Really Means

3. ...And He's Coming Back

The problem for the Losers Club is that fear doesn't just go away - particularly not in a place like Derry with so many ghosts. And because they're destined to forget about Pennywise, overcoming their fears and each other, they're going to find themselves in exactly the same position as they were when the clown first appeared to torment them.

Actually, that's not entirely true. When Pennywise comes back, some of the group will actually be in a more vulnerable situation than at the start of the 1988 period. Bill won't have Richie, Eddie or Stan, and none of the original core of the team will know one another. They will be alone, which is precisely when they correctly identify themselves as being in danger from Pennywise.

Because they're creating the perfect environment for him to come back for revenge, when the time is right (and It has hibernated for the usual 27 years), there's no question that he's going to come back. And Bev's vision of her and the Losers fighting It once more as adults seemingly confirms as much.