I don't see that there is any "load on the ATC system" If you want to fly one of the more popular tracks on the North Atlantic [....] you will be reporting on ADS-C every 4 minutes.
My comment was not a speculation but a fact. The South Indian Ocean airpace is not the North Atlantic , and data link processing load is an issue for the Australian system, hence the trial.
Of course while this means that the regulators can say that they are "doing something" ADS-C update rate change would have no effect on a future MH370 scenario where the 'cooperative' / Active tracking devices are switched off.
There we agree 100%