Why has amplifier power become a big issue

Nothings has really changed.  Loudspeakers with a sensitivity  in the mid-80s will sound perfectly fine with 30-50wpc.  Of course these same loudspeakers will sound better with 150-200wpc when played at louder volume levels.  However, if your typical listen level is 75-85dB, then 30-50wpc is sufficient for most types of music.

It's been my experience that if high volumes levels are required that high sensitivity loudspeakers is a better route than high wattage amps.

I think class D amps are modern marvel, but they don't truly solve the problem.  Feeding mega-watts into a moving coil loudspeaker will result in thermal compression unless you over build the loudspeaker which makes it too massive to properly respond to transients.

A 93dB sensitive loudspeaker with a 50 wpc amplifier will have an equivalent max volume as a 85dB sensitive loudspeaker and a 300 watt amplifier.  I just think the former is a more elegant solution.