Himmel isn't the true hero, as the demon king isn't the true calamaties that threaten to destroy the world. The prove is demon king has been existed for thousand years yet the world never threaten to be destroyed to begin with.
That's why Himmel a "fake hero" is able to defeat the demon king, a "fake calamities". Not to mention the party has Frieren which actually can kill the demon king herself, n the other member just need to protect her.
The sword will be pulled by a true hero that will face the real calamities that will destroy the world, which hasn't happen yet.
But hey, that's just my theory, a manga theory... XD