Personally, no I don't think anyone should be forced to finish something they can't face writing, for whatever reason. Life has wonderful ways of throwing curveballs at us, and when RL is a pain, it can hugely impact the ability to write.

If anyone's desperate for the end to a story, can't they try writing their own? Isn't that at least a little bit of the point of fanfic anyway - to tie up loose ends?

*gets off her soapbox and hides under a rock*