One Hundred Years of Solitude's version of mortality varies depending on who is dying and who is left behind to mourn. Early on, the dead are an important part of the lives of the living: haunting them as ghosts or being dragged along as bones as a form of remembrance. But later – aside from Melquíades, who is somehow able to defeat death – the dying really do disappear forever, especially as they start dying by the thousands rather than individually. Death is shown to be a lonely, isolated condition. Although the dead long for the living, the living soon forget about the dead.
In a world where so much lives on after death (either people themselves, in the form of ghosts; memories of ancestors and their idiosyncrasies; or artifacts and relics that still hold meaning), the only true way for someone to stop existing is to stop being remembered.
Gabriel García Márquez is going against the grain: instead of immortalizing characters through his writing, he is creating them, just to wipe them off the face of the planet.