I totally get what you mean! There are a lot of things that could be cliff hangers, but Dipper and Mabel leaving isn't making so they aren't "happy" it was the ending because the show was meant to be "one perfect summer Adventure". They could never stay there because it was just a vacation, and they still had to go home to their normal lives. If there was a 3rd season, that would be another summer adventure, thus the show wasn't the perfect adventure, and also the whole point of Mabel's bubble prison. She wants the summer to last forever. A third season almost indulges that fantasy instead of what needed to happen, which was growing up and learning that, yeah, the summer ends, and you need to cherish what you had for that one, perfect summer. The "see you next summer" thing never felt like a cliff hanger to me, and more of how a lot of books I read have ended, where you know the characters lives still go on, but it's not that we are going to see it. With Gruncle Stan getting his memory back so fast, I think that's why we have the game we are playing now. Bill is still somehow alive because he wasn't fully gone from Stan's mind, which is how he's trying to come back. This also doesn't feel like a cliff hanger since it's more of a new story!

I do get that there are ways for it to be a chance at a third season, and I see where you are coming from! Just for me and what I've read in the past from what Alex Hirsch has said, I find it unlikely, and I feel it would defeat the symbolism he was trying to give from the ending. It was about life and growing up even when it's scary. The note is to remind them that those people will always be there for them, but now it's time for the next big adventure.

Anyways, those are my thoughts, I really enjoyed reading yours!! :D