The idea isn't that copyrights will expire--we'll all be old by the time that happens--but that maybe his kids or grandkids will value money more than the personal belief of not doing a haunted house. Not a lock by any means.
For all the discussion, I've never seen a direct quote from King why he hates haunted houses so much. It's odd given how open he is over things like The Shining. If it was just not wanting to associate his brand with something cheesy, you'd think the houses approved by John Landis and Steven Spielberg might get him to at least give Universal a shot. Maybe he secretly hopes to do his own event one year? Maybe he had a bad experience as a kid? Or a scare-actor at a local haunt stole his girl in high school? Seriously, if anyone ever meets him at a convention or something, please ask.