I have heard some arguments about Alicorns. This one guy thought that alicorns were only meant for the "gods" of equestria, Celestia and Luna. Then once Cadence was revealed to be an Alicorn, he was pretty taken by that fact. Since Cadence was clearly not immortal or controlling of anything as powerful as the sun or moon so this busted that theory. I think, alicorns are meant for princesses, not just "gods" It all matches, Princess Celestia, Luna, Cadence and now, Twilight.
But here are my questions, If they introduce more princesses, will they be alicorns too? what if a pony is an earth pony transformed into an alicorn? will they acquire both a horn and wings? And I've seen some pretty interesting fanart of Twilight staying young while her friends age and ultimately die before her. (very sad indeed. The feels got to me) if this is true, than Alicorns are immortal, does this make cadence immortal too?