Oh my young padawan.
Dreamworld let the Eureka Mountain Mine Train stand, not operating, from 2006 to 2017. The motivation to do ANYTHING with it was probably born out of the loss of Thunder River and the need to either restore it to operating condition (a ludicrous concept by park management) or to demolish it (as the rest of Goldrush needed to be anyway).
As for not having a new attraction in 4 years - bear in mind that
two of those years was in a pandemic, and the parks suffered numerous shutdowns in that time
Movie World is part of a group - and as long as i've followed these parks, the parks have usually taken it in turns to get something new (although Wet N Wild suffered for a few years from lack of attention)
BOTH of the other 'ride' parks in the chain - SW and WNW are seeing new attractions constructed this year. Atlantis was supposed to be finished last year, and WnW was this year's plan. Which means MW's next attraction may not be far off (although do bear in mind we've seen a lot of refurb work going on too - RoadRunner is about to open with a new train and ride system).
Steel Taipan Vs. Leviathan - putting aside any issues or delays with construction - just comparing the actual ride that will eventually open - 'much better' and 'modern' aren't really any sort of justification. Wooden coasters are being built every year around the world. They are traditionally older technology but that doesn't mean the current rides being built are old.
Each have their own points of difference. I think it will be a very hard thing for even the coast's most astute enthusiasts to categorically state one is so much better than the other - just 'different' and 'in their own way'. It does remain to be seen however - so passing judgment now, whatever the outcome, is foolish.
Anyhow - with all that said - I don't see how you can justify that MW will be declining in 10 years time, simply for the reasons you state. In the past 5 years, Dreamworld killed four people, and removed (or will soon remove) 7 or more attractions, and have only installed two new ones, and yet, with that track record, you're saying Dreamworld is on the rise, and this is apparently and ostensibly because of Steel Taipan? One coaster does not a good park make.
I'll leave you with this - 10 years is a long time.
10 years ago - Movie World opened Green Lantern.
Also 10 years ago - Dreamworld opened Buzzsaw.