Prophesy says that Twilight Sparkle will not outlive her friends. To preserve the Princess, her friends must survive. They may not be immortal - but one year can become a thousand at the speed of light.
A few years after Luna's return it seems that Equestria will finally know an era of peace and appears to be on the verge of a new renaissance. Ponies are happy. Luna is recovering. Celestia is miserable.