That does make a certain sort of sense, but wasn't the entire point of Twilight's mentorship under Celestia to raise an heir to the throne? Would it really make sense to raise an heir with such a relatively short lifespan? This raises an interesting question: Assuming that Twilight now has Celestia and Luna's lifespan, what will happen when her friends die? Will control of their elements pass over to Twilight somehow, or will they have to find new bearers for all the elements besides magic? The series won't expand on this for obvious reasons, but it's interesting to think about.