I'm just going to try and answer each of your questions in order:

I think that John Williams' music is way too iconic to be forgotten, especially since so many people associate his theme closely with Star Wars (which is already a hugely popular movie). Whereas, many people may know that Hans composed music for, say the Dark Knight, but there is less of a correlation between those movies and the Dark Knight theme as there is with Star Wars and its theme.

Hans is already pretty popular, especially among younger audiences, since that is mostly who he caters to (by younger I mean early 20's and maybe late teens). His music is, at times, repetative and loud to the point where it distracts from the movie (interstellar was crazy loud, which I know was done to portray the emptiness and echos of space, but come on...).

John Williams' music is much more "spot on" (for lack of a better phrase) with the movie it is done for. Jurassic Park's theme was perfect for the sense of wonderment that was associated with the movie, while Jaws' theme was intense without going over the top. Hans Zimmer really only has one tone that he tries to recreate with all his movies. Though he is widely popular, I find it difficult to separate his themes from, say Inception and Dark Knight. He uses the same crescendo effect in all of his movies to create a sense of tension.

Both Williams and Zimmer both have very successful scores for their movies. However, I find that John Williams' music is much more important to the movies that he wrote them for. Star Wars main theme, Jurassic Park main theme, ET, Jaws, Harry Potter. Almost anyone you ask can recite the themes to these movies. I deseray to say that you would have a much more difficult time finding someone to recite the main themes from Man of Steel, Inception, Interstellar, but maybe not the Dark Knight.