Arwen's Mortality

[A_Brandybuck]

Another question is bothering me. Tolkien made clear, that Elladan, Elrohir and Arwen had to make their choices. They could delay their choice, but not for forever. But what if they don't choose?

Until the end of the Third Age they have (seemingly) lived like Elves. They have owned the youth of the Eldar and didn't age like mortal men. But they weren't Elves, but Half-Elves. They could live like the Eldar for all the time and delay their choice and delay their choice and delay their choice, if there is no force to choose. But what if there don't choose?

Or does their choice is bound to their behavior? I mean, could their choice can automatically derive from their decision to settle down in Middle-Earth or to leave Middle-Earth. The Elves are fading, the time of the Elves ends in Middle-Earth. Leaving Middle-Earth could only mean to choose to be among the Firstborn, but could staying mean to choose to be mortal?