A lot of the classic music played in the concert halls today is actually ballet or opera music (show tunes). Many have not be performed with dancers or singers in a long long time. Most classical concerts begin with a "Overture" from some pre-20th century show of some kind. Mozarts 21st Piano concerto is called "Elvira Madigan" (the name of a movie is was in).
In their day Beethoven, Mozart, Bach, Brahms, and most of the others were not held up on the lofty pedistal we put them on today. Bach was all but forgotten until Felix Mendelssohn revived his music in the 19th century. Mozart got no respect probably until the very early 20th century. Brahms in his prime was considered too old fashion to ever amount to anything long term.
Rossini works as I mentioned were performed as entertainment at gambling casinos (not the MET like today). Rossini got a fixed fee plus a percent of the house take the night of his shows. Can you image him alive today performing his operas in Las Vegas? Would you predict he would become an opera legend based on that.
Go to the store and you'll find recording by Orchestras around the world of movie and other music by people like Elmer Bernstein, Eric Korngold, Miklos Rozsa, Bernard-Herrmann, Richard Rogers. Even Prokofiev and Shostakovich composed film scores.
There are recordings already of John William's movie music arranged into concert suites, even by foreign orchestras. Leonard Slatkin is on record as saying some of John's marches in a few movies, are as good as anything composed by John Phillip Sousa.