My personal theory re: Nightmare Moon/Luna, as I've said before, is that yes, ponies would die if there was an eternal night. That's what Nightmare Moon wanted. But Luna is to Nightmare Moon as Hal Jordan is to Parallax. The thousand-year lock was the only thing powerful enough to contain the raging fury of Nightmare Moon, and to give Luna a chance to win back some control, which is why the challenges the ponies faced were so carefully tailored to them. Luna couldn't stop Nightmare Moon, but she could give the ponies a chance.

Of course, this is all 100% fanon, but I like to think that there are as many Equestrias as people dare to imagine.