Okay, so what we're interested in is that first tweet. Twilight won't outlive her friends. Okay, so we know alicorns live for thousands of years, ponies on the other hand, do not. Now this could mean multiple things.
1. Twilight isn't a full alicorn or something along those lines, thus making her turning into an alicorn meaningless and even more pointless.
2. The other main 6 being Elements of Harmony gives them the magical power to live forever.
3. Twilight DIES! It says she won't outlive her friends, that means she has to die before them or either they all just spontaneously combust at the same time.
4. Twilight loses her alicorn nature at some point, becoming once again a normal unicorn.
5. The main 6 all become alicorns.
6. Meghan MeCarthy is just telling us what we want to hear and doesn't actually know. I've come to distrust the folks over there at Hasbro, so I wouldn't be surprised.
So that's some of the possible options that this tidbit could entail. What do you guys think?