General techniques against worms might include: isolated networks, host diversity, rate-limiting, and traffic anomaly detection.
Are these low-cost/high-return existential reduction techniques?
I can't imagine having any return in protection against spreading of AI on the Internet at any cost (even in a perfect world, AI can still produce value, e.g. earn money online, and so buy access to more computing resources).
Your statement sounds a bit overgeneralized - but you probably have a point.
Still, would you indulge me in some idle speculation? Maybe there could be a species of aliens that evolved to intelligence by developing special microbe-infested organs (which would be firewalled somehow from the rest of the alien themselves) and incentivizing the microbial colonies somehow to solve problems for the host.
Maybe we humans evolved to intelligence that way - after all, we do have a lot of bacteria in our guts. But then, all the evidence that we have pointing to brains...
ITT we talk about whatever.