I agree, that Williams was lucky in many ways. But it’s not just luck. Obviously Williams proved to have a strength in character and manners that led people to confide the lead of the Boston Pops to him and not to someone like Goldsmith who had a reputation of being kind of a diva.
And even on his original soundtrack albums Williams presented dedicated concert arrangements of some of his major themes. I think, Goldsmith never did that. The best he did was creating special album edits for the album presentation.
Oh wait, I think, Chinatown has a dedicated album arrangement of the main theme. And Pappillon, too. So, maybe I am wrong about this point.