I don’t think it’s fair to say that Rumpel will never change. We’ve seen examples of Rumpel expressing love and emotions after he became the DarkOne. Even though he was consumed with its evil power, he still was able to find a place in his heart for Belle. Regina, on the other hand, hasn’t shown any emotion or capacoty to love after she became evil. We thouht we saw a little emotion in her in “Fruit of a Poisonous Tree,” but then that turned out to be a trick. We thought we saw it a little bit in “The Thing You Love Most,” but then that turned out to be a trick when she slaughtered her own father in cold blood. Again, we thought we saw it in “Hat Trick,” but that was a red herring too because we all know what Regina eventually ends up doing to her father: Murder.
At least after he became the Dark One, Rumpel didn’t murder anyone he loved. Regina, on the other hand, repeatedly caused harm to those she claimed to love the most. Her father, the Genie…
Big difference.