If the combination of loudness and time got too long for a particular receiver(or separate amplifier)and it started to overheat, the protective circuits would simply shut it down and the amplifier wouldn't suffer any permanent damage. The overheating possibility has no effect on the speakers; there the requirement is clean power, and the amount needed has no necessary connection with impedance. Sensitivity of the speaker is a determining factor, and the M80s, for example, are slightly above average in sensitivity and require less power than many speakers with an 8 ohm nominal rating(such as my M22s).
Sure, enough maximum power capacity for a particular use is needed, but in the vast majority of cases typical receivers with power ratings in the 100-200 watt(only 3dB difference)area are more than enough. Unused headroom is simply that: unused.
