I would have to disagree there, your saying that just because she doesn't "see" her friends die, means that she doesn't outlive them? I'm sorry but if her friends die before her, no matter when or where.... she will have outlived them...
There are two possibilities because of this:
1) Twilight, although an alicorn, is mortal.
2) Twilight IS immortal, and her friends are somehow immortalised as well, just as Twilight was...