Historically I would have agreed with you. But since the rise of LLM-assisted coding, I've encountered an increasing number of things I'd call clear "ghost bugs" in single threaded code. I found a fun one today where invoking a process four times with a very specific access pattern would cause a key result of the second invocation to be overwritten. (It is not a coincidence, I don't think, that these are exactly the kind of bugs a genAI-as-a-service provider might never notice in production.)