Not all good things can last forever, and in some cases, it's actually good, in turn, for them to end at a certain point.
I really miss a great deal of my favorite shows that have ended in previous years, but if they had gone on much longer, Lord knows what could've happened to them. The results could be anything from major dips in writing quality, to soulless money-milking that extends the given series' lifespan without giving it the care and heart that was once put into it, causing the franchise to lose it's meaning that made it special and lovable in the first place, to a mix of all of the above.
Alex Hirsch ended Gravity Falls when he did simply so that he could avoid any possibility of letting his show fall into that territory. Do I think it could've gone on for another season or more, and still give us brilliantly written stories and characters like it always has? Yeah, but when you see so many other formerly great cartoons get extended lifespans yet lose so much of their quality in the process, you can completely understand and respect his decision. And it's not like the series spanning only two seasons caused the story to feel rushed or crammed, either. It's pacing was handled well and it ended at a good time and on a high note. Just another reason why I love it so much, and though I miss it immensely (did I mention it's my favorite show of all time? Yeah, I think I did), I'm fully satisfied with the entire story it told, and I'm glad neither the show itself nor the team behind it lost their magic at any point; it was thoroughly enjoyable all the way through from the beginning to the end, and I can't say that about a lot of fictional works anymore, animated series in particular.
Sometimes a franchise can continue for a long time without issue...other times, it's best the people involved with it's creation, writing and funding end it at a certain point, on a high note, and not try to drag it out to tell more stories and/or make more money, as much as people may crave to see more of the world and characters they've come to love. Every story out there has an ending, after all. When you try to explore the story's world past it's ending, there may not being anything left of it to enjoy. There may be nothing special left to tell. These fictional worlds and their inhabitants may go on in our imaginations, but there gets to be a point where that spark they once had disappears and the experiences they could go through are, for reasons that vary, ultimately no longer intrigue or interest us. There's a point where there no longer any more stories left that these worlds or their characters can tell...at least, not any that have the same impacts they used to, or at least, as great of an impact as it's conclusion had. Sometimes, it's best to just let something good end when it really needs to, no matter how much we love it, and no matter how profitable it is/was for the parties who took part in giving it life.
I guess it's kinda late for me to make a post like this, and it's also probably kind of out of the blue, but I just felt like sharing some thoughts I had since there was talk about franchises we love and their inevitable conclusions earlier.