> Why do Elon Musk's speculations have any credibility here? It's not his field, and even in his field he has a track record of making confident public statements that turn out to be wildly wrong.

Actually it's very much his field. Elon Musk was one of the founders of X.com which eventually merged to become PayPal. He was interested in digital currency since the '90s and has been active in the cryptocurrency space as well.