> The fact that neural networks have been the only way we've been able to solve a lot of problems, which approximate the way that the human brain learns, is pretty strong evidence of this

These are all difficult assertions to make because we're trying to prove a negative. Your claim is evidence that neural networks are one way to do it, and nobody's arging that. But it's not evidence that there doesn't exist a better way, or that alien life might evolve a different and even less optimal way.

Legs are pretty ubiquitous, even flying insects have them. If we'd never seen a fish or snake we might conclude that they're inevitable. Somebody that evolved to be rad-hard on a planet without a magnetosphere might conclude that life can't exist without the thick carapace that they're made of and only look for planets rich in silicon and calcium.