Maybe they have a future at water parks :
I've seen some maximum weights quoted here but not sure if they take in to account that once a pax aircraft is cannibalised in to a freighter all the uneccessary weights are stripped out of it allowing for increased payloads to be carried.
While it's true that you save weight by getting rid of the passenger accomodations, most of that weight goes back in on a freighter for things like the cargo handling system, side cargo door, and stronger floors. So the max payload numbers don't change much.