Accidents happen in theme parks. But they're usually accidents. If you put aside the 'rider was an idiot' incidents like standing up in a log flume, or jumping a fence into a ride envelope to retrieve a hat, the vast majority of incidents, and especially deaths has come from inadequate maintenance and safety protocols.
Disney has killed people - the big thunder incident was attributed to disney downsizing its inhouse maintenance team and outsourcing the work to external contractors who didn't do as good of a job.
Dreamworld's incident is by and large due to their arrogance. They had a head of engineering who wasn't a qualified australian engineer, and they relied on him to guide what was necessary. When incidents occurred (such as the constant failures of the conveyor) they took his advice to remove slats from the conveyor to lighten it. Never mind that every rapids ride in the world that used a slat based conveyor had closely spaced slats. Dreamworld treated the symptom. Conveyor failed? lighten it's weight - without risk-assessing what the possible impacts of that were - the gaps in the conveyor were now wide enough to accommodate a raft tube. The proper fix would have been to replace the conveyor motor with something more powerful to overcome the weight issue.
Just like the station rails, that were introduced to prevent rafts dipping while loading, without considering what impact that might have to a stacked station.
But they had many opportunities to fix the issues. Several flips occurred in the ride's lifetime. But each was put down to an operator not following the manual, and when they identified issues, they simply instructed the ride operators to do more themselves to prevent it, rather than introducing controls.
Right before the accident they were in the process of upgrading systems, but they chose to install anti-rollbacks on the conveyor instead of a sensor system to stop the conveyor when the station was occupied. I really struggle to understand how they saw a conveyor rollback as a bigger issue TBH - even if there was a raft at the bottom of the conveyor, and a raft on the conveyor slid back down and bumped it - would that have been more than a bumper-boat scenario? they installed rails at the bottom of the conveyor to prevent rafts bottoming out - so at worse, you've got a shunt situation? The only thing I can think of is that they felt the bottom of the conveyor was more of a risk because it was outside of the operator's line of sight, whereas issues in the station could be seen by the operator - again, placing another burden on an already overworked and stressed operator.
So many single things could have been done which would have prevented this. Just one of these things would have changed the outcome -
installing a proper, power disconnect e-stop.
not installing rails in the station, instead using rams to pinch the raft against the station (similar to Snowy River).
leaving the slats on the conveyor, and instead beefing up the motor that drove it.
installing sensors to detect the presence of rafts on the conveyor and in the station to control-stop the conveyor when the station was occupied.
Replacing the pumps that kept failing, and not operating the ride until it was fixed.
Everyone, park management included, saw this ride as tame, gentle, and unable to hurt anyone. It was arrogance, especially on behalf of one particular department head, that caused this.