I’m actually a little surprised Marvel finally caught on to this matter and acted before it was too late.
The original choice to list the two Avengers films as a two-parter was something I fully expected would kill interest in the MCU all on its own. What works so well with the MCU is how each film is stand-alone, yet they’re all loosely tied together. If you don’t want to watch Guardians of the Galaxy, you don’t have to.
The two-parter approach screws up that style a few ways. It forces people into a commitment of more time and money if they want the whole story. The first film essentially gives you an idea of what you can expect with the next one, meaning there’s much less “new” about it. A year’s gap is ample time for people to lose interest in the story/characters, and forget what happened in the first film.
The formula of multi-parters only really worked for things like Hunger Games and Lord of the Rings because they’re based on books. Book-based movies are a beast all their own.
Now that they’re not really retitling the films, but splitting them to seem more separate, I’m not sure about my original prediction that the MCU would be dead by 2020. There have been several problems along the way, like failure to make Black Widow merchandise or the race matter in Dr. Strange casting, but I don’t know if they’re big enough issues on their own.