Thermal Distortion your loudspeaker most likely suffers from it. But do you care?

As Atmasphere described, cost was a big driver to higher power and lower efficiency.  But, it was also partly a result of the success of stereo.  When it was just one box, it wasn't quite so bad that the box was big in size.  But, when stereo came along, there was a big push toward making smaller speakers.  Smaller size meant lower efficiency, but, that tradeoff was now possible because the transistor made it possible to get more power relatively cheaply.  

 I posted because I feel many over-fixate on known or obscure issues without realizing nothing is or can be perfect all is flawed

I guess Easter week is as good a time as any to get philosophical, but even my almost 40 year old speakers have sounded close to perfect for 3+ decades.

Or at least they offer a close enough taste, to convey the perfection of the thoughts.

but even my almost 40 year old speakers have sounded close to perfect for 3+ decades

I'll wager that just replacing the caps would astound you. Nothing fancy, just polypropylene for electrolytics and Mylars [<<--- N A S T Y]. Erse and Dayton are plenty good.

Solder any and all push on connections.

I know because I BTDT for decades... 😎

If your speakers come from the likes of Spica, buy a couple of extra of each cap and match* as closely as possible to stock films. 40 year electrolytics are going to have an ESR that's likely off the charts.

* Requires a capacitance meter or a pal with one.

But prior to all that, the industry moved away from field coil to permanent magnets, not because permanent magnets were better, but because they were cheaper as well!

That is correct.