> This phone doesn't have the latest and greatest software/hardware (version locked at iOS 12), but it does its job
I hope you do not browse the Internet on it, since a lot of security bugs are probably not fixed in its browser (and you're not allowed to install another one). So you put all your data on the phone at risk when using the Internet.
Personally I expect with the release of iOS 15 we'll probably see this happen. Though Apple is the one with the knowledge of how many iPhone 6 and earlier phones are active in the field (assuming telemetry), so decisions would probably be based off that.
iOS 12 is currently in the n-2 slot, but even on macOS older versions have gotten security updates as needed.
I expect updates it to last one or two more years given their track record. And even when it stops for months, they always come back for emergency security updates.
I hope you do not browse the Internet on it, since a lot of security bugs are probably not fixed in its browser (and you're not allowed to install another one). So you put all your data on the phone at risk when using the Internet.