Let me point out that we (humanity) does actually have some experience with this scenario. Right now, mobile code that spreads across a network without effective controls on the bounds of its expansion by the author is worms. If we have experience, we should mine it for concrete predictions and countermeasures.
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?
No, these are high-cost/low-return existential risk reduction techniques. Major corporations and governments already have very high incentive to protect their networks, but despite spending billions of dollars, they're still being frequently penetrated by human attackers, who are not even necessarily professionals. Not to mention the hundreds of millions of computers on the Internet that are unprotected because their owners have no idea how to do so, or they don't contain information that their owners consider especially valuable.
I got into cryptography pa...
ITT we talk about whatever.