This is explored at length in the books. The wizard supremacists are an allegory for race supremacists, and it’s pointed out repeatedly that they’re delusional, broadly impotent, and small-minded. The death eaters believe so completely in their godhead and his divine right to rule that they neglect to consider even the most basic countermeasures or alternatives. There’s some “death of Stalin” paranoia but it’s overwhelmingly unforced incompetence.

Voldemort, likewise, blocks out even the potential for his lessers to outdo him. He is utterly infallible and his insecurities as an incest orphan to a fallen house will not allow him to feel any differently. Meanwhile, he’s been repeatedly circumvented and beaten by a group of teenagers, and his brewing plans will lead to the complete genocide of wizardkind upon reaching the world stage. It perfectly encapsulates the way race supremacists view the world and why they fail so frequently. The enemy is both strong and weak.

Darn shame rowling’s turned into what she hates. She wasn’t terrible at writing populism 101.