That sentence should read something like the most important popular music figure of the SECOND HALF OF THE 20th century.
There were guys like John Philip Sousa, Paul Whiteman, Benny Goodman, the Dorsey Brothers, the Gershwin brothers, Cole Porter, Rodgers & Hammerstein, etc etc etc, who dominated the first 50 or 60 years of the century.
Sousa was a 19th century figure. His last popular song was composed in 1909. And he was not noted as a performer.
Whiteman was neither an exceptional performer nor a notable composer, either. He was skilled at arranging other people's music and he had an unsurpassed nose for talent. Anyone who was smart enough to hire Beiderbecke, Trumbauer, Venuti, Teagarden and Berigan is a great man.
The Dorseys were important bandleaders, but they were very much of their time. Sinatra made recordings with the Dorsey Brothers in 1947 that sold millions. He made recordings under his own name in 1994 that sold millions as well. That's a career that spanned 50 years.
The Dorseys enjoyed a decade as popular hitmakers then faded into TV orchestra work and obscurity before Tommy died.
The Gershwins and Porter were songwriters who contributed dozens of the very best songs to the best singers' repertoires - but they themselves were not famous as performers.
Rodgers and Hammerstein were also not performers. Their collaborations were brilliant - the standards by which musical theater is judged.
Basically you are arguing that the most prominent composers/lyricists/arrangers of what is now known as The Great American Songbook are getting short shrift.
However, the greatest and by far the most popular interpreter of the Great American Songbook was Frank Sinatra.
Millions of Americans know Gershwin, Porter, R&H, and the Dorseys solely or mostly through Frank Sinatra's performances of their work.
"There were guys like John Philip Sousa, Paul Whiteman, Benny Goodman, the Dorsey Brothers, the Gershwin brothers, Cole Porter, Rodgers & Hammerstein, etc etc etc, who dominated the first 50 or 60 years of the century"
Bless you for saying that and how true. Rodgers and Hammerstein's music and lyrics can never be equaled. They were one of a kind.