>>There's a big question on whether these approaches work well for onboarding the next billion users to the internet
That isn't what captchas are used for. There are two side to every captcha, what the website is using them for and what the captcha creator or provider is using them for. In google's case, captchas are being used to hone AI. So it is no wonder that items like hydrants, store fronts, crosswalks and traffic lights dominate. Those are the objects that any AI-controlled vehicle will have to recognize. Given that the first market for AI-driven vehicles seems to be western nations, specifically the US, it is no surprise that google gears its captcha program to objects from that area.
If google were developing robots to pick fruit captchas would ask us to identify bananas and oranges. I worry about the day that captchas start asking me to differentiate people from other objects. I don't want to help train an AI to tell people from trees, soldiers from bushes.