If any evidence that you ever existed is deleted when your existence is devoured, what happens to any children you may have had?
They stay, but lose all their memories about you. Except when someone takes your place like Shana does once, which seems a good way to easily get a family.
But if everything about you was erased from existence, why are people like Shana or even her enemies able to remember them at all? If you ate someone by wiping them out from existence such that even their past is gone, you'd forget that you'd just ate somebody! There'd also be nothing left in the universe for you to even BASE a torch on, and people would immediately forget about the person even WITH a torch. It's logical snarls like that that had me call bullshit on the entire series.
Knowledge of the Crimson World is apparently enough of a buffer to remember people who've lost their existence. But by all means, get indignant about things you haven't bothered to learn.
Yuji isn't real. He doesn't exist. Even if his MacGuffin allowed him to live forever, that just gives the universe more opportunities to take the MacGuffin away. He WILL vanish from the world. No one will remember him. Shana KNOWS that he's not a real person, but just a convincing shadow, and yet she allows herself to forget this. What a fool. The series having a central character who isn't even really a person is one of the other reasons I call bullshit on the whole series.
The relevance of "isn't really a person" was discussed. Very early on.
During Yuji's fight against Shana, he reveals the nature of "Silver" to Margery Daw to stop her from breaking through his barrier. At first it appears as a generic Kick the DogBreak the Haughty moment, but when you think about it, she would've found about "Silver" later anyway, and her contract would've ended then because she would've been devoid of purpose; that way, she can continue without pursuing Silver. Granted, he still did Kick the Dog, and break her he did, but it wasn't really the worst thing he could've done, which reveals the high probability that Snake of the Festival isn't as bad as he appears.
Power of Existence is a metaphor for influence upon reality. All the magic of the series is accomplished by influencing reality. Love is the strongest Unrestrained Spell because when present in a person, love has unlimited influence on their being!