The main issue with this is that Apple doesn't allow downgrading of iOS. Thus, one might be able to maintain a device on an older version of iOS that is most su

The main issue with this is that Apple doesn't allow downgrading of iOS. Thus, one might be able to maintain a device on an older version of iOS that is most suited to it, but if anything happens that requires a restore, then that device is effectively dead in the water. It seems to me that at the very least, Apple should backport relevant security fixes at least one major version, if not two (for example, a bug found in iOS 11 should also be patched in iOS 10, should it be present), and they should sign the last revision of one or two major versions back, so that these things aren't thrown out en masse.