Funny you should say that about Superman, from having operating it and many of the MW rides it honestly is one of the more reliable rides in the park. It wasn't very common to have ride shutdowns, Scooby and Green Lantern I would say are the 2 least reliable rides in the park (well they were when I worked there)
I can't speak too much for Arkham, but when it was Lethal it was a very reliable ride and was very rare to ever see that ride go down. I really hope that whatever is wrong with Arkham is able to be fixed and it has another few years (5) until they can work on what's going to replace it.
I'd say the Giant Drop maybe has another few years at the most in it's current incarnation as it's had it's fair share of downtime over the last few years. I would say an upgrade would be more likely though. The Claw would probably have another 10 years in it's lifetime before it would need to look at being replaced.
Hot Wheels Sidewinder (or whatever it's going to be called) honestly will likely have a fair amount of time left in it. It's a fairly simple ride as far as mechanics go, and it's not insanely forceful so it wouldn't have too much stress put on it. And for many of it's early years it didn't really operate. I'd say it's got another 10 or so years left in it, and honestly I think it might even outlast the MotoCoaster and not for reliability but because it's a better ride IMO. I even think it MAY outlast BuzzSaw as that thing is a problem child from what I hear.
So for Dreamworld, I think the next major rides to go will be:
Giant Drop (3-5 years - For a new ride experience)
Motocoaster (5-10 years)
BuzzSaw (5-10 years)
Hot Wheels Sidewinder (10 years or so)
Movie World:
Arkham Asylum (2-3 years at most, unless they fix all the issues)
Scooby Doo (5-10 Years)
Superman (10 years - Maybe longer)
These are just my predictions, but could be wrong. But I don't think the Sidewinder will be the next Dreamworld ride to go personally, especially if it's getting an overhaul and the way it weaves in with WhiteWater World.