The key here is that Avengers: Infinity War appears to represent the culmination of a decade-long storyline.
Much of Infinity War’s core storyline remains firmly under wraps at this point, but if nothing else, it seems pretty clear that Marvel has done an excellent job of selling this project to an eager audience.
This movie will provide a climax for every Marvel narrative planted since the debut of Iron Man in 2008, and all of the marketing for the project seems to promise a strong sense of finality — even though Avengers 4 will follow one year later on May 3, 2019.