There's a reason that On the Waterfront isn't called Terry's World or The Ballad of A Guy Who Could'a Been A Contender.
And that's because this isn't just Terry's story. This is about a community that rallies together to take down a dude named Friendly…probably because they were peeved that he chose such a ridiculously inappropriate name for himself.
This film doesn't follow one character exclusively or only show us things that Terry is aware of. It checks in with different people during the course of the story: showing Edie talking with Father Barry or with her father, dockworkers bantering with each other, Johnny sending Charley to deal with Terry, a rich banker deciding not to do business with Johnny...
It gives us a picture of Terry as a courageous hero, but also lets us see his whole society—the ways in which it helps him, and the ways in which it works against him. It tells the narrative of the docks themselves, and of who's going to control them and determine the workers' fate.
In other words, it's about the scope on life on the waterfront.