The following contains spoilers for Avengers: Infinity War. You have been warned.
When Avengers: Infinity War was first announced, it was presented as one story broken over two movies. The third and fourth Avengers movies were even to have Part I and Part II in the titles. However, as time went on, Marvel began to seriously downplay this idea, before eventually refuting it entirely, claiming instead that while the two films would certainly be connected, they were separate entities. Now that we've seen Avengers: Infinity War, there are many who feel that, just as the original titles indicated, we only got half the story. However, that isn't really the case. Avengers: Infinity War isn't half a movie, it's a complete story. It just has another story to tell next year.
To be clear, Avengers: Infinity War absolutely ends with plot threats left free and fans dying to know what happens next, but that's simply not the same thing as cutting a movie in half. Today, with movie franchises being the order of the day, that's standard practice. Even before the current influx of franchise-focused movie making, sequels happened and often they were set up in advance. Even if the MCU wasn't planned out for years, it would have been clear to anybody watching Captain America: Civil War or Thor: Ragnarok that there was going to be another chapter following what we saw there. Certainly, the implosion of the Avengers or the destruction of Asgard wasn't the end of the story. We knew that Loki's impersonation of Odin in Thor: The Dark World would be dealt with at some point in the future. There was clearly more to come, but nobody accused these movies of being incomplete.
Nobody would claim Star Wars: The Empire Strikes Back was half a movie, and that's probably the closest analogy to what we got with Avengers: Infinity War. In both cases we've seen our heroes be essentially defeated, some characters are taken out of the game, and we have little idea what they'll do to come back. But we still know they will be back. There was no question at the end of The Empire Strikes Back that we'd get The Return of the Jedi. Just because most in the pre-internet audience didn't know the release date, and it hadn't yet been filmed, as Avengers 4 has, didn't change that.
It's true that for a lot of favorite characters, things are less than clear. Those that are still with us now sit frozen in time as we wait a year to see what they do next, while others, for all intents and purposes, are dead. The thing is, that doesn't really matter for the story that Infinity War is telling, because the movie isn't about them.
Avengers: Infinity War is a complete movie because it tells a story from beginning to end -- the story of Thanos. We follow Thanos (Josh Brolin) on his quest to obtain the Infinity Stones and use them toward his goal of universal "balance." The movie ends when it does because that story is now over. Thanos has achieved the goal and, as the final shot of the film shows us, he's done. We assume the remaining Avengers will be coming after him, and the Infinity Stones eventually, but he doesn't. He believes he's won. It's over.
The ending isn't even a cliffhanger. The damage has been done. Certainly, we'll have questions over the next 12 months as we wonder what will happen next. We had the same questions about how Han Solo would be released from carbonite. It wasn't even a question of "if," but "how" -- and we're asking the same question here.
When Avengers: Infinity War is compared to something like the Harry Potter and the Deathly Hallows movies, which were broken into clear and distinct parts, it's obvious what half a movie looks like. The first part, not unlike Infinity War focuses on a villain trying to collect objects of power, and the heroes trying to stop him. However, in the case of Harry Potter, Voldemort doesn't even obtain all the Deathly Hallows before the end of Part One. Those two movies are truly designed as two halves of a complete tale. You could watch them without interruption and they would fit together nearly seamlessly. Infinity War has a clear break. There will almost certainly be a jump forward in time between Infinity War and Avengers 4, it may not be a great one, but with the damage done there is no sense of urgency that you get with a cliffhanger ending. To be blunt, nobody is going to get any deader while the Avengers try to figure out what to do next. There's no reason for the movie to pick up right where it left off.
Avengers: Infinity War leaves a lot of questions, but it's supposed to. There is absolutely another story to tell, but the Infinity War is over. Thanos won.