All there really is is arguments from theory (most notably, the orthogonality thesis and instrumental convergence). It's not like we have a spare universe to conduct experimental research on the likelihood of human extinction. That said, the 5-10% range is not some specific person's wild guess, it's the median wild guess of a poll of AI experts: https://aiimpacts.org/2022-expert-survey-on-progress-in-ai/
> or they have some weird counterintuitive agenda (e.g. Anthropic and OpenAI trying to position themselves as the amazing, trustworthy keepers of this dangerous technology before their IPOs).
They are not "trying to position themselves" that way - back when those companies were founded, the people who did it were already convinced they were trying to make the ("inevitable") development of AI go better. It's not some novel galaxy-brained marketing strategy, it's just what they thought the whole time.