There's preservation societies out there who are passionate enough about steam trains that they'd maintain and run both the former and present steam trains on-site with very little to no hard labor costs. I'm not a train buff by any stretch, but steam trains and seeing that kind of old school technology in action is badass, and just like Dollywood's steam trains (or Disneyland's) there's a ton of education and community engagement opportunities to be had by having the steam trains run as often as humanly possible at Dreamworld. I'd say the best-case scenario is for the diesel train to relieve the steam trains during say Monday-Wednesday and for the rest of the week the steam trains are running with regular school programs, select tours and other programs that would help to further augment the opex of the steam trains.
It's also worth nothing that it's good for the brand. Just like what Al Mucci did with DWF globally or Marty Ermer did with Dreamworld Corroboree, Dreamworld as a brand has so much soft power to be gained by participating with local and international communities. At the minute I think revenue wins out over brand or culture, hence why these people are gone, their programs are gone (or minimised) and just like the steam trains, the horse-drawn carriage, the glass-blowing, the bakery etc. etc. etc. all of these influencing have been killed over time because with Ardent, sadly, money above all has ruled the roost for a number of decades now.