If a signing certificate expires and is stolen, it can backdate signatures. On the theory that expiration is useful because either people keep poorer track of k

If a signing certificate expires and is stolen, it can backdate signatures. On the theory that expiration is useful because either people keep poorer track of key material over time or algorithms get weaker over time (which is not an unassailable theory, but it's a coherent model), you want expiration to prevent future use just as if it were revocation. Because you can't trust the date of a possibly-forged signature, you have to check the current date. The model you're suggesting is closer to the "timestamping" one commonly used in code signing (IIRC Windows and Mac both do this) where a third party that's particularly trusted to handle key material well long-term gives you a second signature over the message "I saw this signature at this time" (effectively they are analogous to a notary or witness for real-world signatures). Then you can trust that signatures from expired signing certs were actually made in the past, and not by an attacker who got hold of the key. That is, without timestamping you have no proof of #5 in your list. (I suppose you could do this now for the SSL PKI with Certificate Transparency logs.... it isn't exactly what they were built for but it's probably sound.)