The issue is the way unions work (or at least my understanding, correct me if wrong) is that in many places once a union has taken hold, they actually get the right to prevent a non-unionized worker for working. That's absolutely crazy to me (and evil, and inefficient).
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Note: The above doesn't mean that Elon doesn't treat his workers like shit - he does. I wish people would see through the cult of personality with him, because so many workers have sustained injuries on the job or just burnt themselves out for someone who really doesn't care about them.
The thing with trying to save humanity from existential threats, is ethically speaking you can justify sacrificing as many actual humans for the cause as necessary.
But yeah, unions aren't the mechanism to fix that.