
Why dont they just chop off philip's head or something

If they'll just communicate then Lucia will know why he can't produce or something, but that's not the point here. All of them, expect for Lucia doesn't know the reason why she's infertile, they don't even know how to cure her, but Lucia does. She could make herself fertilr again which makes Philipp irrelevant.
Technically he isn't. He's just someone who remembers his past life.