One thing that I can imagine about Twiley having longer life than the other Mane 6 is just like Simon (or Viral, if she's immortal) in Tengen Toppa Gurren Lagann. Kamina died but Simon finally was able to move on, then later the other guys including the entertaining twins died, followed by Kittan, and Simon managed to accept their passings -- just like Viral who had to accept the deaths of his superiors which included his crush/someone he admired as one of his superiors and also the death of The Spiral King himself (who was supposed to be immortal).
Now, coming back to the MLP story -- yes, I know Twiley will be able to handle her friends' passings just like Simon and Viral, because she's a strong-willed alicorn, after all!
But she's not going to outlive them so I'm guessing that they all die TOGETHER due to a plague or an apocalypse..... dun dun dun!