If you increase the difficulty much beyond what it currently is, legitimate users end up having to wait for ages.
The reason why anubis works is not the PoW, it is that the dev time to implement the bypass takes out the lowest effort bots. Thus the correct response is to keep the PoW difficulty low so you minimize harm to real users. Or better yet, implementing your own custom check that doesn't use any PoW and relies on ever higher obscurity to block the low effort bots.
The more anubis is used, the less effective it is and the more it harms real users.