So a powerful Rumpel is a Dark One, and a powerless Rumpel is a Woobie. Is there really a happy ending here?
Yes, because even a powerless Rumple is a dangerous, dangerous man. Even if Rumple chooses Belle over power and TLK breaks his Dark One curse, consider that Rumple will still have the following, in any realm:
Lots of gold
A 300-year old collection of dangerous magical objects and the knowledge of how to use them
People who owe him favors/have deals and you do NOT break deals with Rumple
The Cane of Pain ™, which he’s used on people twice his size
The ability to manipulate everyone, is smarter than 99% of Storybrooke
Remember that Rumple set fire to a castle with molotov sheep, shivved the original Dark One, kidnapped and whalloped Moe, kidnapped and roofied Princess Abigail, beat Hook half-to-death, beat Keith to strawberry jam, shivved Pan in the back, burned Regina’s mansion, manipulated a sheriff’s election, and released a demon into Storybrooke WITHOUT MAGIC (and therefore without the Dark One curse but that’s another argument). Homeboy Rumple don’t play.
Things that sound bad even IN context
So don’t worry, Rumple fans– even if he’s depowered he can still beat you into next week with his bare hands and his angry floof.