I didn't find it surprising at all, really. How are they going to top Bill Cipher destroying the world through chaos and L'il Gideon's Heel-Face turn? I hate saying it, but I'd rather the show go out on top than struggle to keep itself at that level. And Alex himself wanted to end things after Weirdmageddon, the show did have signs. What were Dipper and Mabel going to do? They were going to go home at the end of summer, and the show's reached the end of summer. And it's not like it ended because Disney wanted more money poured into Pickle and Peanut. Thank Grob for that.
Of course, I'd love to see some way the characters can live on via spinoffs, a movie, or comic books or something. Or better yet, merchandise you can actually find in stores. I'm talking to you, Jazzwares Gravity Falls toy selection.