What if... the monsters are like viruses and the Magical Girls are the Earth's immune system. Metaphorically speaking.
Magical Girls exist because Monsters exist, not the other way around, essentially a reverse Superhero Paradox. The earth is always a little bit sick with monsters and until it can be cured (if it can be cured) it keeps generating Magical Girls. This also ups the stakes on Cassandra because not only is she a mass murderer, which is bad, but she's wrong on such a catastrophic level that if she accomplished her goal she would basically kill the earth.






