The two trickiest things to pull off on TV are a good beginning and a good ending. You need a great start in order to hook audiences, and then a good ending so as to not sour the entire experience. But even if the first episode isn’t 100% great, there is a chance to course correct in the second or third episode. But an ending? There’s no coming back from that, just as there’s no convincing parents who named their kids Daenerys that they didn’t make a horrible choice. As good as the “Dexter” revival season was, there is no erasing that horrible final season — which fans were so disappointed with that they temporarily turned the r/Dexter subreddit into a “Breaking Bad” one during its final episodes.
The importance of an ending is especially true of animated shows, which don’t always get the benefit of a proper ending, since cartoons are usually aired out of order and therefore not granted serialization. There are exceptions, of course, which make them even more special. Shows like “Avatar: The Last Airbender,” “Phineas and Ferb,” and “The Legend of Korra” all added something spectacular, and delivered series finales that served as both satisfying conclusions and also as good episodes in their own right.
The “Gravity Falls” finale in particular is interesting because, like Alex Hirsch said, it leaves some dangling threads while mostly focusing on the emotional arc of the characters — which it does, spectacularly. This is not at all dissimilar to how shows like “The Leftovers” and “Lost” treated their finales, focusing on ending the emotional journeys of the characters rather than answering every single lore question.
Source : https://www.slashfilm.com/1239926/alex-hirsch-left-the-ending-of-gravity-falls-intentionally-vague-in-case-he-wants-to-return-to-it/