You ever stop to think that the rule was made by the Allfather as a kindness to mortals who remember their beloved dead?
Yes, some of the Einherjar were berserkers when living, but many were just ordinary men who were
good soldiers and did their jobs and died as good warriors should. When they come back as Einherjar, they won't be as they were, we've seen how they fight. When Murphy comes back, she will not be the Murphy of Harry's memory, no matter if he lives forever or not.
That is a kindness to him, who remembers her as she was, and spares him confronting what she has become or will become, because it won't any longer be Murphy.