There is the consideration on how much more delayed the passengers would have been if not rebooked onto an earlier service

The poster indicated a 3 hour delay in Sydney due to weather

I would be surprised if the airline put passenger on an earlier flight without agreement .

This wasn't just a downgrade on a normally operating flight , but delayed flights due to weather, rebooking to try and get the passenger home and an oversold cabin

It seems odd that the airline would rebook and downgrade without any involvement of the passenger

The passengers could have refused to board in economy class and waited for the original flight I suspect