You can claim that Marvel might be over-saturating the market and I won't argue, but your comment about their films losing money at the box office and having to make it up in merchandising is false. They are all profitable (very profitable). The latest, for example, Captain America Civil War, had a 250 million budget, but made 1.15 billion at the box office. The merchandising just makes it all the sweeter for them.
http://www.the-numbers.com/movies/franchise/Marvel-Cinematic-Universe#tab=summary