Everyone saying "they gotta bring in Fantastic Four and X-Men" are forgetting that Marvel made hits out of B and C tier characters like Captain America, Thor, Iron Man, Guardians of the Galaxy, and Black Panther.
They did it with:
1. Great casting. Downey, Hemsworth, Evans, Boseman. These were all once in a lifetime castings. Even who they got for Loki and Star Lord was crazy good. Their charisma carried a lot of movies. None of these new people are touching that.
2. The overarching infinity stones story kept us interested even when the movies weren't so good (Thor 2, Iron Man 2 and 3)
3. Every now and then they'd deliver a legitimately good movie. Not a good "comic book movie". But a movie that stands up on its own like Winter Soldier.