Regulation.
Make it illegal to request the content of a webpage by crawler if a website operator doesn't explicitly allows it via robots.txt. Institute a government agency that is tasked with enforcement. If you as a website operator can show that traffic came from bots, you can open a complaint with the government agency and they take care of shaking painful fines out of the offending companies. Force cloud hosts to keep books on who was using what IP addresses. Will it be a 100% fix, no, will it have a massive chilling effect if done well, absolutely.