No one knew about Manticore. No one cared because no one knew. The ones who knew...

[He shakes his head, looks down.]

Charles Xavier said that it's important, when people die for something they can't help or because they stood for something they believed in, that other people know about it. That the people who were there tell others about it, so the death isn't wasted.

So the person doesn't disappear forever because no one knows and no one cares.