I don't see the absolute equivalence here. An annual pass to our parks costs only $200, or $180 for 2 weeks, so if you're going on a trip purely for a theme park it really can be quite cheap.
I guess cruises do compete with parks as holiday choices, but in the same way cruises compete with EVERY other type of holiday you can think of in the same way. It's no surprise people are picking cruises more often these days, having done the Royal Carribbean South Pacific cruise myself, it's brilliant value for the hospitality you get from staff on board, seeing beautiful parts of the world every few days, as well as all of the on board entertainment, buffet breakfast every morning, restaurant dinner every night, the people you meet (and don't forget the plentiful excuses to drink). If you wanted to experience all of those things on the Gold Coast, you'd be paying quite a bit more, but if you're prioritising the parks over everything else, and make sacrifices in other areas, it'll end up being the cheaper holiday, it's a lot more complicated than just comparing the two holiday choices I think.