I'm out of pocket for a day or so (writing commitments) but thought you might find this 1959 analysis of the big band era's demise interesting and quite valid:
"Ellington and Basie were not the dominant figures of the late 1930s; Fletcher Henderson, as arranger for the enormously popular Benny Goodman band, was becoming the most influential musicians of all. Goodman played his arrangements; Shaw competed with similar ones; Edgar Sampson wrote them for Webb; Gifford wrote them for the Casa Loma; Oliver for Lunceford and, later, Dorsey; Durham for Basie and later, Lunceford. The increasing popularity of swing arrangements on the Henderson model led to a general similarity of style in all the big bands. Goodman, Shaw, the Dorseys, Barnet, Hines, Calloway, Teddy Hill, Webb were all approaching the same standards of proficiency. There is a terrifying record, an anthology called The Great Swing Bands, on which most of these bands are represented. If they are played without consulting notes or label, it is impossible to distinguish one from another.
It is customary to date the decline of the 'band business' after the end of World War II, but, artistically, swing had died earlier. By the early 1940s the gradual elimination of stylistic variations had killed big-band jazz. It was a death by entropy."
—Hsio Wen Shih, a reviewer for The Jazz Review, from the book Jazz: New Perspectives on the History of Jazz by Twelve of the World's Foremost Jazz Critics and Scholars (1959).