So, I was just thinking about what would have happened if Nightmare Moon had won and brought eternal night to Equestria.
Well, I thought that no day = no sun = no heat, meaning that it would always be cold.
With that, plants would eventually perish, since they can't live without sunlight.
Even if ponies managed to live a while without plants, they, along with every other living being, would eventually die of starvation due the lack of plants and oxygen.
Eternal night would become an eternal darkness of death, with no one to witness the 'beauty' of the night, as Nightmare Moon desired.
It seems like she didn't think that one very well...