> Likewise captcha is useless against multimodal LLM available to anybody, many of which can be run locally

You have a fundamental misunderstanding of why captcha exists. If the attacker has to deploy a multimodal LLM to solve captcha, the service provider using captcha has already effectively achieved its goal - since the goal is to raise the cost (in terms of any combination of complexity, computing power, and dollar amount) of sending bulk traffic and ultimately to reduce, not eliminate, the overall attack rate. 0 bulk traffic is never the goal.