> Also, what's with the ever-shrinking renewal periods? Why can't a certificate be valid for 10 years?
Also, why can't a certificate be valid longer than the certificate(s) above it in the verification chain?
For code signing certificates that is how it works. When checking a given link in the chain for validity the time check is whether or not the upstream certificate was valid at the time the downstream certificate was signed.
It is analogous to how notarization works with physical documents. A notarized document does not cease to be notarized just because years later the Notary Public who notarized it retires lets their notary license expire.
For HTTPS certificates the check is that each certificate in the chain is valid now. In physical document terms it is like if when a Notary Public retires it invalidates all their past notarizations.