“… I haven’t heard that name in over 5000 years”

I prefer the “Peter B Parker” treatment

“No. I’m the other purple alicorn.”

“…”

“Of course, I am!”

Old, bitter, exhausted, and defeated, with a whole bunch of scars as reminders of past conflicts. Everything was taken away from her, because the very ponies she was trying to protect turned against each other and her. Lost confidence in the power of friendship, because she learned the hard way how fleeting, temporary, and fragile it really is. Her friends’ passing didn’t help. Twilight’s body is immortal, but Twilight the person, who she was, was so intimately connected to her environment and relationships that it ceased to exist once they did. Because a Princess of Friendship is an avatar of interpersonal relationships, things that are far more temporary than the sun or the moon.

Who’s to say that this alicorn will even answer to that name? “Twilight Sparkle died a long time ago,” she might say. “Twilight is in the Beyond with her beloved friends, to laugh and play forever. Me? I don’t know who I am. I shouldn’t even be here. My world is dead. But yours is just beginning. Cherish it, because it won’t be here forever.”

This can go one of two ways: She fakes her grumpiness and embraces a new student, or she’s just down-right “The Last Luke Skywalker” unpleasant.

“No, I’m the prince of Yakyakistan. How many other purple alicorns do you know of?”

If she ever did visit her, she would have that rainbow hair.