the golden apple tree grows a new apple every time one of Dipper’s descendants were supposed to be born, had he not been turned into Alcor. and a new bough for each new family that was supposed to exist (the trunk being Dipper and formerly fated spouse, 1st branches; their children, 2nd splits; their grandchildren, etc.) maybe it’s a peek into a universe where Mabel instead had become a demon? the idea pains and relieves Dipper; that he lives this life, and that Mabel won’t have to. not here.