I guess I just don't see what's stale about any of these. The characters and storylines are meant to intertwine, so it feels like one really big story with a lot of fun chapters. I even read the comics these movies are mostly based off of and felt the stories were pretty original, but like I said earlier, maybe I'm just too much of a fanboy.
I can see it both ways. People who loved the comics/characters are of course going to love the universe-building and the stories. Your analogy is a good one - a big story with lots of new chapters... but that's only interesting as long as you are someone who wants new chapters.
But, there are a lot of people on the other side who are looking for a new book. My wife for example is pretty bored of the Cap/IM/Thor centered Marvel movies now. I'm kind of hitting a little bit of a wall too, which is why I probably enjoyed Ant-Man and GotG more than Avengers 2 at this point and why I'm looking forward to stuff like Black Panther or Dr. Strange a lot more than Cap 3 or Thor 3.