Basically RumBelle have a 1 in 4 chance of a happy ending so why can’t A&E give them some happy time whilst they are alive together?
Because RumBelle serves the narrative function of being the foil to all the other “happy” couples and Rumple serves the narrative function of being the stand-in villain until something bigger and badder comes along. Rumbelle is the token dysfunctional “don’t do this at home” pairing. Also, they need to be broken up for the redemption via offspring trope they might try to do relative to Rump. Whether that will work out for him, I’m not so sure — even in the best case scenario, on OUAT a character who has a child and wants to do right by him/her can be quickly kicked to the curb in some gruesome, absurd way in order to then give said child to another, morally objectionable character, as a way of humanizing them.
I think RumBelle is bound for bad news either way — I suspect they’ll have Rumple die in some self-sacrificial manner yet again, especially if it’s true that Bobby wants off the show after s6