There are a couple of lists (odious as they are) of "top film composers" he ranks highly on:

Top 10 film composers – of all time from BBC's ClassicalMusic site and Screen Rant's (never heard of it before) The 10 Best Film Music Composers Of All-Time, Ranked but both lists contain only very recent composers, making them very short-sighted.

When you look at just a few of the other "serious" composers who worked in film, the fact that Williams has made such a great living at it doesn't make him any less of a serious composer than the rest in this august group. It means he found a niche that has kept him very busy.

Aram Khachaturian wrote amazing stuff (my favorite symphonic piece from films is his work on Spartacus), Leonard Bernstein wrote several things, not just West Side Story, Dmitri Shostakovich was a very busy composer for film, George Gershwin, Bertolt Brecht, and the list could go much longer. This is a non-folk or blues music subject, I realize. The "other" John Williams pops up around Mudcat, and he's a classical guitarist, so there is wiggle room for these other folks.

There are composers like Sergei Rachmaninoff whose music appeared in lots of films (and he died in Beverly Hills in 1943) but he never actually composed for film. So there is that distinction to be made. Those listed above made a good living writing for film.