Adam wrote out the argument of why networked software should expire, but he backed away from the conclusion I had proposed: we need some sort of hard failure mode where after some expiry date it's guaranteed a given generation will be gone. We're accustomed to the idea that mechanical (and even more so, biological) devices will eventually break down, and even with mechanical computer parts (hard drives, power supply fans) we know we need to replace them. Why not solid state hardware, like networking switches?
Or even: why not software? Users would revolt, sure, so it's more of a rhetorical question. But the internet is now filled with literally millions of zombied Windows boxes in botnets that will continue attacking people until hopefully the power supply fails. What's scarier is rootable networking hardware (like a consumer switch) that are all solid-state, with nothing to fail.
(This idea is actually due to someone other than me or Adam, but I can't remember who I got it from, sorry... if I had to guess I'd say Graydon.)