Genndy always wanted to end the show with a movie, right?
He had such a vivid vision for a movie that he prefered to just leave the show open ended when making a movie became impossible, right?
I'm open to the possibility that the whole point of the guardian and the portal was just to have them be destroyed in that movie too, to raise the stakes. I mean we have seen it a million times in fiction, give the audiences hope so you can then crush it and make it hurt harder, but also make wathever victory the characters can find more meaningful.