From my understanding even if you're lobotomized your body's physical make up would still eventually decompose like… a lightbulb in a room left on, or an engine without maintenance, it will eventually ware itself down and ultimately cease to function. In this case death would eventually come to those whose "hearts" are snatched.
I feel that Madoka's world balances out entirely with the witches world. Madoka formulated a world that would equally spread out suffering and not concentrate it to little girl tears, now it's everyone's tears. A wraith does not immediately kill so it is more likely to save people rather than an instant inescapable death from a witch or her labyrinth.