I think you can rate limit without tying it to IP address. If each page returns a session key only valid for the next page request, then you force bots to wait as long as you want and/or spend money on extra memory for parallel sessions. One problem with this is, e.g., if people come to your site from an indexed link and have no possible session yet. In that case you probably would want to add a delay after some amount of requests per IP, so you'd slow Tor users down but only on the first request to your site. If your page is JS or browser dependent in some way, then bots would probably need about 100 MB per thread. All of this is in the ballpark of paying people to solve captchas.

This was a problem before Tor anyhow. You can run a proxy for a few cents a day.

I just don't see how captchas are some awesome solution. In any anti-bot technology, the cost to circumvent it is pennies. It strikes me more as something like DRM which just makes content producers feel good, but really only punishes average people.

edit: sorry I hadn't read your links. Good points and hopefully someone like CloudFlare would make this easy for people to add to their sites.