I notice that my go-to AI chatbot, bing.com/new, now often complements its answers with some unrequested links (complete with pictures) of related products I can buy. Seems more like affiliate marketing than ads at this point, but shows that free AI, in chatbot form anyway, has a potential to generate ad revenue. Nevertheless, I agree that a non-free API will be the biggest revenue stream simply due to the volume of hits on an API.