That, and the fact that LLMs are now available to pretty much anyone for effectively nothing, would make me very cautious in basing my judgement of something being a scam or not exclusively on a caller's accent, spelling, mannerisms etc.

Lately, it's actually been quite the opposite in my experience, and I don't find that too surprising either: A lucrative scam business can afford to pay much more than the average US company that sees customer support as a cost center to be optimized at any cost. So why wouldn't their staff's English be better?

Social engineering scams are about to become a lot more exciting (in a bad way), not least thanks to LLMs (with and without voice capability), and I think people are absolutely not ready for it, not even us professionals working in tech.