The problem to me seems that it needs to scale with the number of requests. So PoW effectively burns energy. As the problem with captchas, is rather who profit

The problem to me seems that it needs to scale with the number of requests. So PoW effectively burns energy. As the problem with captchas, is rather who profits from them and who could track users, it seems that with an OCR system to be protected, it is most reasonable to actually give OCR tasks to humans. IMHO this is far more sustainable and people would understand the value: there is nothing bad in improving ML in general. Maybe someone could even define a sensible PoW task for OCR but I doubt it...