Argument: Harder to keep the populace in line, if everyone has more power.

Taking what Baughn has said further, you could postulate that the normal 'pony's life is about <cutie mark concept>' is inverted for alicorns: Concepts are .."embodiments" of alicorns. By which I mean that, .. Applejack has a life which is about apples, whereas the moon and stars' existence and state is about Luna. This would imply that every alicorn needs phenomenal self-control and forebearance to avoid disturbing the structure of everyday reality. (and also perhaps that the sun, stars and moon had 'previous owners' and behaved differently.)

That may be -too- powerful an argument, even if it doesn't pertain directly to death. It is somewhat vulnerable to Jossing, as I'm sure in the next two episodes there will be some discussion of what it means to be an alicorn.