It didn't really seem worthwhile starting a new thread. I was on the QF127 SYD-HKG in business a few days ago with a severe delay. Originally scheduled at 10:20, helpfully at about 6am it was moved to 11am. Less good when at the gate at 10.30 it was moved to 11:30, and then whilst queuing to board the pilot came to say "Sorry an engine fan has been damaged". This required an aircraft (and pilot, but not crew) change - so whilst waiting in the lounge the flight time was moved to 16:30... So overall a 6 hour delay, and an arrival into HKG at 1am rather than 6pm. This was even less fun as I was with two kids under four. I did get a minor improvement though in that we were moved from a 20yo A332 to a 23yo A333 which has slightly better seats I think.

I did rather cheekily send a request for an EC261 level of compensation but in QF vouchers. More quickly than my flight delay, I got a very rapid "sorry but we told you about the delay so you don't get anything" email response.

From reading threads here it seems like that's it really, I may be able to claim a bit of a travel delay insurance - admittedly I didn't have any extra costs (I just enjoyed the SYD F lounge, and entertaining the kids), but it just makes me think I will never be silly enough to choose QF over CX again on the route. I presume forum regulars would advise me to not waste my time with any further comms?