Loudspeakers have we really made that much progress since the 1930s?

John what exactly has been forgotten?

We keep perfecting the wheel too. Once something basic is discovered, it's discovered.
But, yeah, the spirit of creativity is down everywhere. Rudimentary computer digital times. Sucks.

I don't think we have lost the ability to innovate- the goals and priorities are different. When those magnificent early theatre sound systems were designed, they were addressing the needs of an entire industry transitioning to the "talkies" and trying to accommodate the space limitations of existing theaters. But, the resources were there and the top companies were competing to develop product. 
Today, look at big data- the ability to collect and analyze massive amounts of data and apply it- to everything from serving targeted advertising to national security- is where the action is, and there are no doubt innovations in that field.  The trickle down to consumer level products- in the form of "smart" phones and appliances, as well as interfaces that are guided by past user selections is what we get, not necessarily better audio. (Though things like DSP have made woofer set up for modest home theatres pretty easy).
Materials science and acoustic models (aided by computers) may have improved, but some of the materials- copper in field coil speakers- mercury vapor tubes, or even the materials and tooling for most vacuum tubes are expensive or hazardous or obsolete. (Look at what happened during the "vinyl boom"- suddenly there was a need for record presses-and none had been made for years; now, those presses have been salvaged, rehab'd and new ones are being built simply because there is demand).   
I'm fascinated by the early days of audio. I'd love to see and hear your private museum at some point John. 
One book worth reading although it isn't strictly technical, and has some gaps- is Cowboys and Indies. Dumb title, but it is essentially the history of reproduced sound and the emergence of the industries and businesses that depended on it (music publishing, record companies and trends in listening behaviors from the 19th century to date). In the process, there is some discussion of the changes in audio technology and how that related to cultural changes at the time. 

Mapman whats missing-  realistic sound quality, the ability to allow listener to feel the emotion of the music. The ability to easily hear the mix ie pick out the individual instruments and vocals along with the added studio work. Realistic image size and dynamic range.The at ease at any SPL the ability to sound wonderful out of sweet spot and through out home. Extreme lack of listening fatigue. A clear real sounding vocal ability the ability to do this all on massively low power. Extreme reliability and ease of service in field by owner. Today most all of this is missing and if present only a small part of it.