Why do you think they started updating system/apple apps outside of the iOS version releases. They can keep updating the apps and give security updates to the older version of iOS for previous gen phones.
Do they actually do this? AFAIK, almost all the system apps are bundled with the OS, and don't recieve any updates after the next version is released; the last time I remember them ever releasing a security update for an old version was the certificate validation issue a few years ago.