As early as the February 1939 issue of
Downbeat, Duke Ellington warned that swing was becoming "stagnant" and added that "It is the repetition and monotony of the present Swing arrangements which bode ill for the future. Once again it is proven that when the artistic point of view gains commercial standing, artistry itself bows out, leaving inspiration to die a slow death. The present dearth of creative and original music is not, I'm convinced, due to a lack of talent."
https://books.google.com/books?id=Zc4Lh9KC2MIC&pg=PA23
The swing bands were certainly the closest thing to jazz-as-popular-music America has ever known. (Though one should remember that they--especially the white bands--did a lot more pop than really "jazz" material. And even Ellington sold a lot more records with, say "Sophisticated Lady" or "Satin Doll" than with the jazz masterpieces that wowed the critics, like "Ko Ko."
) Their decline is due to various reasons. James C. Petrillo's recording bans, especially the first (1942-44) one, are often blamed. I have some reservations about that theory, as I once posted here:
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Suppose Petrillo had taken Selvin's advice and not called the strike. Or suppose the record companies had been less confident they could outlast the union, and had agreed to Petrillo's demands in July 1942. Here are some of the real and alleged results of the strike which might have been avoided:
(1) The most tragic result of the ban was of course all the great recordings that were never made. Just to mention the 1942-43 Earl Hines band with Charlie Parker and Dizzy Gillespie (and the band's new "girl singer", Sarah Vaughan) should be sufficient to make that point.
(2) The real winners were the small independent labels which settled with the union before the majors did. In fact, Capitol Records, a relative newcomer, achieved major status in this way.
(3) It is sometimes argued that the strike resulted in the downfall of the big bands and the rise of singers (who, as noted, were not subject to the ban). IMO this simply accelerated a trend that would have taken place anyway. Already before the ban it was notable that singers like Sinatra were becoming more popular than the bands (e.g., Tommy Dorsey's) with which they sang. Not only were big bands becoming too expensive, but there was a growing market for other forms of popular music--rhythm and blues (among urban blacks) and country music (among southern whites). (The development of country and r & b was enouraged by BMI's breaking ASCAP's near-monopoly on music publishing--I'll get to that in another post someday...) Furthermore, the younger jazz musicians found the big bands too restrictive a framework for improvisation, and developed the bebop style which was essentially (despite Dizzy Gillespie's big band) a small combo music.
(4) Gunther Schuller, in
The Swing Era: The Development of Jazz 1930-1945, p. 847, arguing against those who suggest that "the recording ban broke the momentum of jazz's progress" has claimed that in some ways the ban actually helped jazz. In part, this is because he thinks the big bands were largely becoming stagnant, so their displacement by singers in pop music, and by small combos (which were free to pursue more adventurous music) in jazz was not ultimately a bad thing. Furthermore, he argues that:
(a) The new bebop style may have been fortunate in not having to contend in its formative years with a booming record industry, "which surely would have rejected it or tried to muzzle it creatively. It was not until new small record companies sprang up in the mid-1940s, companies either willing to take some risks or actually supportive of the new jazz, that bop found a friendly haven in the recording field. And by then bop was strong and ready for its unrestrained diffusion."
(b) Record companies began to reissue their older recordings, "which was not only a boon to jazz record collectors but created an awareness among many music lovers that jazz
did have a venerable history to be taken seriously and appreciated, at the same time reminding listeners of the deeper values in the pre-Swing Era jazz, values which had been in many cases dissipated and abandoned." Indeed, there was a revival of interest in traditional jazz around this time, and the ban may have played a role in encouraging it.
https://www.alternatehistory.com/forum/threads/wi-no-petrillo-recording-bans.451057/
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All in all, I think it was inevitable that the younger and more adventurous jazz musicians would turn to more complex music--but there was obviously a price that had to be paid for that in terms of popularity. And if they had just gone on playing as they had in the late 1930's, the public would have gotten tired of it anyway--there is always a public desire for something at least superficially "new" (as long as it is sufficiently in the old groove to be easily understandable).