I actually think it is unlikely that all the main princesses will have their happy ending. I read that the makers of the show were going to kill Charming off in the first episode, but other members of the show were not happy about this so they didn’t do it. It is sort of like they were going to kill of Jack in the first episode of LOST, but that didn’t feel right so instead they had the smoke monster kill the pilot. I think that is very likely that some of the main princesses don’t get happily ever afters and if i were to pick one it would be Rumple and Belle.