I wonder if this is because some people keep their iPhone 5s around as a backup phone or for some other reason?
My iPhone 5s is still attached to my apple account so a certificate update is probably useful security-wise? But that doesn't seem entirely likely because Apple's account automatically degrades the level of access depending on the age/model/OS version of the device.
More likely that, the Support Contact Rate (and potentially legal contact rate if the phone gets fully bricked and unable to make basic phone calls) is higher than the cost of just pushing the certificate.
My iPhone 5s is still attached to my apple account so a certificate update is probably useful security-wise? But that doesn't seem entirely likely because Apple's account automatically degrades the level of access depending on the age/model/OS version of the device.