Life is tricky, fiction is trickier.
We don't know if Meg is immortal, or even whether she will outlive Elizabeth. She hasn't aged, so she probably won't start aging — but she has no guarantee against death or injury by disease, accident, or violence. Like the cliche says, you might get hit by a bus. You can worry about outlasting the ones you love, you can plan against a sad future, but you can't live there. Life is short even if you won't die.
(My wife is a dozen years older than I am, and her health conditions — and mine — make it pretty likely that, barring mischance, she'll die first. So I find this kind of fictionalized examination of those issues helpful, really.)
