Although it's easy to do so, a movie should not be judged solely upon the way it ended. Of course, the ending is the last thing we are left with once the movie ends, and it is also what we become most invested in during the course of the story. We become invested in the outcome. How is this story going to resolve itself?
Part of what keeps us watching a movie is if it keeps us asking, "what's next?" Once we stop asking that question, we stop caring about the movie and may or may not finish it.
All of that being said, we have to remember that the movie must end at some point, and if every story ended with a nice little bow on top, then it would be boring. As much as we may not want to admit it, we need to be deprived of a happy ending occasionally. Otherwise, what's the point?
Granted, some movies do have crappy endings or just seem to fall flat at the end. (One example that comes to mind is The Bad Seed from 1956. It's a great movie, but the ending, although it makes sense on some level, just seems to come out of left field.) Others, tho, need to end exactly where they have ended. Of course, we still have questions, but if the director took the time to answer all of those, then you'd be trapped forever, especially considering that some answers lead to more questions. This isn't reality TV. This is a movie with a beginning, middle, and end. You may need to use your imagination and form your own theories about what happened after.
As the popularity of "found footage" films has risen in the past few years, I hear people groan at the endings sometimes. Take a step back and think about the concept for a minute. This is found footage. Why did someone "find" it? Why wasn't the footage turned into a studio and made into a documentary? Well, I'm sure that was the director/documentarian's plan, but that plan obviously went awry. I mean, for this footage to have been found by someone else, the person filming the footage must have met an unexpected end. If that's the case, then where they met their end is where the filming would stop. If filming continued beyond that point, it would be contrived. Sure, you might get more answers, but at what cost? Your answers would cost you the last 90 minutes you've been watching since the documentary feel is now lost.
What makes these found footage movies good is that they feel like a real documentary was attempted. For 90 minutes, this movie has felt like someone was trying to make a documentary. Now this person dies. If the antagonist/possessed person/serial killer picks up the camera and decides to keep rolling, then it's just not the same. Plus, how would the footage be found?
So next time you're watching one of these movies, as you begin to sigh or groan or exclaim "WTF?" when it ends, take a moment, catch your breath, and remind yourself that a different ending would either be too much or may work to negate what you just enjoyed for most of your viewing time.