It is known that no magic is forever; if the Spell of Immortality (initially cast by Cosmic Crown the First over all of Equestria) is reversed, everypony's physical form will decay. From this point, there is no return, and the Cycle of Life will resume.

There was no question: it was better to live. Then why did it feel so empty, living here forever? Twilight was faced with a difficult decision; once made, it would affect not just her, but everypony she knew and had come to care for. Was it truly better to live in ignorance, or was that just an empty solution? Knowledge is power and truth is empowering. Did that make everypony alive not only ignorant, but weak--too weak to face the truth?

Either way, Twilight had a decision to make. The indecision and lack of knowledge only made her that much more determined to find a solution. And so she trekked on.