Most rooms don’t need acoustical treatment.

I agree! All of my systems have been setup in cluttered rooms! Yet the sense of stereo space, depth and impact was undiminished!

Surprise- you're right. Sort of. Most rooms do have lots of different kinds of furniture in lots of different places, which do serve to diffuse and absorb and break up a lot of the worst flutter echo one would have in an empty room.  

But that is about it. And notice, it's all random. Because of this it does no good to talk about "most rooms" because no one really cares about "most" they care about "their" room. As soon as we move from the hypothetical home of the scoundrel to the particular home of the individual then acoustic treatment can suddenly matter a lot. 

The physics and acoustics of standard acoustical treatment are very well understood. At the end of the envelope where extreme performance is paramount the science verges into art. But the science behind things like first reflections, arrival times, and bass attenuation traps is solid. It works. As does a lot of stuff much harder to explain. But you might want to keep in mind that just because we cannot explain why something works, does not mean it does not in fact work. Happens all the time. Can you explain how your car works? Yet it does. 

I think it’s very well understood how a car works.