When I think back to my older heavier amps from Ashdown, Ampeg, Trace, Peavey, Laney, Hartke and others I have forgotten for the moment, they quite often printed peak power on the rear as well as the official continuous RMS rating. The peak output was often two or three times the RMS rating and probably allowed for transient peaks for the occasional slappy bit. Maybe this extra energy was available because of the general over engineering of the older heads (big transformers, capacitors etc)?
I know cost has always been important but nowadays the race to market the biggest output for the price seems to have led some manufacturers to quote maximum RMS figures but maybe the class D topology doesn't allow the peak capability that we were brought up with? Hence the proliferation of 1000 watt class D heads that possibly give us a similar transient load capacity to the 350 to 500 watt heavyweight heads?