There is nothing in Secure Boot that prevent people from running their own software. You can update the Secure Boot DB/DBX with whatever you want. Yes, the certificates expire - my computer was bought 4 years ago and Microsoft's UEFI CA will expire in 4 years. At that point I will probably have bought a new computer, but if I have not, I can update the certificate to the new one they released.
Secure Boot is very much an improvement over non-secure booting, and Authenticode signing is an extension of that security to enable signed-only software to run.
Pegasus.