So the running theory, for me at least, was that while sealing away her sister for 1000 years (possibly forever) was a terribly painful decision for Celestia, the only alternative was to watch ponykind suffer under the cold and dark of eternal night. At the very least, ponies would be hard pressed to survive, total extinction wasn't unthinkable. The only option was to seal away her sister before ponies started dying. And yet, in the finale... we see that with Nightmare Moon in charge, ponies are getting along just fine. Eternal night doesn't seem to cause any problems, Dash and Rarity have jobs they're satisfied with, Nightmare Moon isn't hurting anyone... Everything seems pretty okay to be honest. And sure, we only see ponies inside Nightmare Moon's castle, we don't know how they're doing in the rest of Equestria. But are we supposed to believe that the elements of loyalty and generosity would be sitting pretty in a castle serving a princess who makes that happen? My point is, rewinding all the way back, is there any reason Celestia couldn't have just... waited for Luna to get over her tantrum? Maybe it would work, maybe not, but at the time Nightmare Moon wasn't trying to rule Equestria or destroy her sister or hurt anyone. She just wanted ponies to appreciate the night and went kinda crazy to do it. If we see that no one was getting hurt by this, doesn't that mean that Celestia sealing her away was actually a hugely unnecessary overreaction?
In the Season Four opener, we see the actual transformation and fight. The problem wasn't that Luna/NMM wanted eternal night, but that she was actively attempting to murder her sister.
Well as a counterpoint, in the Season 4 opener we see that Nightmare Moon ever once mentioned eternal night, so whatever.
I think with her refusing to lower the moon, it was just sort of assumed. She did transform into an evil pony of darkness, so it's possible that Celestia just guessed her intentions after the fact. My theory is that Celestia didn't have full control over the Elements, and wasn't able to remove the evil like the Mane Six did. I'm guessing that she decided to seal away Nightmare Moon until the ponies that could fully use the power of the Elements appeared.
