> a two-way partition

These are localized problems, not common-mode failures. I'm talking about a problem with something that takes out a significant portion of the internet. This isn't about a simple network partition; a common-mode failure is a problem in something that is common to an entire class of devices.

A totally contrived example might be a worm that bricks routers (we've seen a few router vulnerabilities recently) and spreads with the speed of the fastest worms. Good luck finding "usable parts" of the internet when a large part of the routing capability needs to be physically replaced and reconfigured. This risk has grown in the recent past with the centralization of many services.