They are perfect predators only because they simply power scale us through raw mana and the like.

They've evolved to mimic humanoids to fool them. They're not just about magical power. That's why I said they're the "perfect" predator of humanoids.

the Killing Beam (can't remember its name) was evolved in Frieren to turn most demons into mere threats.

I think they called it Killing Magic. Which was something incredible during the war and humans turned it into "just standard magic". But as the arc with the city showed Demons aren't just mere threats. They are an existential threat as they play right into the weaknesses of humanoids.