Honestly, certificates should never expire or should expire daily. If certificate revocation works then its pointless to have expiring certs. Its just a mechanism for CAs to seek rent. If certificate revocation doesnt work then certs need to expire super frequently to limit potential damage if compromised. A certificate that expires in 20 years does absolutely nothing for security compared to a certificate that never…
Exactly. Certificate expiration has never really been about security. It's purely for practicality, so that CRLs won't grow without bound. This is especially true now that we have OCSP stapling. From a security perspective, a short-lived certificate is exactly equivalent to a long-lived certificate with mandatory OCSP stapling and a short-lived OCSP response, but the latter is much more complicated. And in this case…
Expiration is not “just” about cryptographic risk either; there are plenty of operational risks. If you’re putting your server on the Internet, and exposing a service, you should be worried about key compromise, whether by hacker or by Heartbleed. Lifetimes are a way of expressing, and managing, that risk, especially in a world where revocation has a host of failure modes (operational, legal/political, interoperability) that may not be desirable.
As for Root expiration, it’s definitely more complicated than being black and white. It’s a question about whether software should fail-secure (fail-closed) or fail-insecure (fail-open). The decision to trust a CA, by a software vendor, is in theory backed by a variety of evidence, such as the CA’s policies and practices, as well as additional evidence such as audits. On expiration, under today’s model, all of those requirements largely disappear; the CA is free to do whatever they want with the key. Rejecting expired roots is, in part, a statement that what is secure now can’t be guaranteed as secure in 5 years, or 10 years, or 6 months, whatever the vendor decides. They can choose to let legacy software continue to work, but insecurely, potentially laying the accidental groundwork for the botnets of tomorrow, or they can choose to have legacy software stop working then, on the assumption that if they were receiving software updates, they would have received an update to keep things working / extend the timer.
Ultimately, this is what software engineering is: balancing these tradeoffs, both locally and in the broader ecosystem, to try and find the right balance.