iOS 15 will continue receiving security updates for a while yet, so there’s no pressing reason to upgrade. Or you could donate the phone to Apple in case they can melt it down and reuse it instead of shredding it.
iOS 14.8 was released on September 13, 2021, iOS 14.8.1 on October 26, 2021, and then nothing else.
iOS 15 will be getting patches even if iOS 14 and 13 don't.
You're treating the exception (12.5.6) as if it's the rule, but it's not.
https://en.wikipedia.org/wiki/IPhone
C-f for "Supported OS release" and there's a chart showing which devices support which OS. 12 was "sticky" in that it was the last one supporting the 5S and 6. 15 will be similarly sticky given the range of devices (6S/SE(2016)/7) that will be stuck with it and will probably get several more years of security updates as a result.