Really? Look at any TV personality (or band for that matter), they'll keep going for as long as they are relevant/paid to do so. If they love what they do (which Derren does) they will continue - even when they aren't on TV. His live shows are incredible (can't wait for Miracle in a few months).
But even if he were to completely disappear in 5 years time, he isn't like the usual 'IP'. He could even be a fictional character. Look at Dr Frankenstein's Scream Machine - a fictional character - still had relevance (and would do today if it were still there). It could have been called Dr Merlin's Ghost Train, as long as we get a background/insight into who Dr Merlin is/was then the ride will make sense. Of course, "Dr Merlin" wouldn't sell tickets as well as "Derren Brown" in the first couple of years, but in year 5/10 (if the IP has left the public eye) it wouldn't matter.
Think Saw: The Ride. When that opened, it sold many tickets. Now, who goes to TP to go on SAW because it's the movie-themed coaster? You go because of the ride. It could have been called Dillan: The Ride and people would be saying NOW that they want to go to TP to go back on Dillan, but that wouldn't have generated the hype back in 2010.