There's a few ways I could see this playing out:
A) The other mane six become alicorn, thus gaining the extended life/immortally Twilight now has.
B) Twilight reverts back to her unicorn body, and thus can live out the life span of a normal pony.
C) Twilight isn't a true alicorn like Celestia and Luna and is not granted extended life/immortally.
D) The other mane six will be granted extended life/immortally through some other means like age spells.
F) I don't know muffins or something.

The show will be cancelled at some point, and consequently their whole world will die at once.