In 1849, Richard Wagner declared, with his usual assurance, that “the last symphony has already been written.” Beethoven’s Ninth, with its eruption of voices in the finale, had, in Wagner’s view, exhausted the form and inaugurated a new age of music drama. The pronouncement went unheeded. In the decades that followed, Brahms wrote four symphonies, Tchaikovsky six, Dvořák nine. After 1900, the idea that nine symphonies represented an outer limit—“He who wants to go beyond it must die,” Schoenberg said, speaking of Mahler’s unfinished Tenth—fell away. Shostakovich produced fifteen symphonies, Havergal Brian thirty-two, Alan Hovhaness sixty-seven. As of this writing, the Finnish composer-conductor Leif Segerstam has generated two hundred and eighty-six (having passed Papa Haydn more than a decade ago, with his Symphony No. 105, “Pa-Pá, Pá-Pa-Passing . . . ”). Composers have also exceeded the seventy or eighty minutes’ duration that was long considered the maximum. Brian’s “Gothic” Symphony lasts almost two hours; Kaikhosru Sorabji’s “Jami” Symphony, which has yet to be performed, would go on for four and a half hours; Dimitrie Cuclin’s Twelfth, also patiently awaiting its première, might devour six.
Ross notes the symphonic cycle that has had the best overall case for being an addition to the symphonic canon has been (surprise, not) the Shostakovich cycle.
And certainly I love some Shostakovich but as symphonic music that people listen to composed since about 1974 or so we could probably have a small consensus that John Williams' Star Wars soundtracks have gotten a lot of play time.
So in a sense Wagner's declaration did come to pass, symphonic music eventually shifted to being a type of narrative drama ... in a way.