This is a pointless and infinitely losing arms race. LLMs will learn to use hyphens instead of em dashes, and so what? You’re going to start using em dashes again?
At this point, I believe the LLM "style" is on purpose. Perhaps the labs want to be able to distinguish their slop from human content for future training; I don't know. But it feels very deliberate that they've kept the style so consistent for so long.