I think it's specifically a combination of 2 things for Apple, one of which Apple will absolutely never admit to, but it's something that I've definitely notice

I think it's specifically a combination of 2 things for Apple, one of which Apple will absolutely never admit to, but it's something that I've definitely noticed back when I still got Apple devices. I was into jailbreaking at the time and that often meant deliberately not upgrading the main OS, which often made my phone last much longer compared to others of the same model who were running the last version of iOS. The first is that Apple forbids you from downgrading the iPhone to a lower firmware. They're not the only device maker that does this, but this is mostly unique in the realm of "personal computing devices" - I can downgrade most Android phones to a prior Android version through fastboot if I wanted to and usually the only thing blocking you from installing older Windows versions is a lack of driver support for them (same deal with Linux distros). Even macOS at least in the past used to make sure that if you booted from the Recovery partition, it'd contain the installer for the version of macOS you got with the computer. Apple claims it's in name of security, which is probably true (although I do believe that a company like Apple should just maintain at least the last two major versions of it's software at a bare minimum and let users freely move between every version they've ever released - voided warranty is acceptable for this), but it leads into the next point. Which is that I suspect Apple deliberately "overspecs" the burden of running basic iOS with newer devices. Basically, the iPhone 4 (yes I'm going back this far) last supported iOS 7.1.2, but if you read reports at the time, the actual most performant version of the phone at the time was iOS 6, not iOS 7, which was mostly characterized as being a slow mess on iPhone 4's that Apple only barely patched up to not be device crippling. I've from personal experience seen this pattern repeated over and over until I just switched to Android (with the main reason for doing so being the fact that Apple killed a bunch of older phone games I was playing by killing 32bit support in iOS). By doing this, they can essentially force people to move to new devices, simply by making the last supported version borderline unusable for regular use. "Batterygate" was essentially the main consequence of this practice, but it was a symptom coming from Apple's support practices, not the cause. It's just something they boiled the frog with (because after the iphone 4, it stopped being reported, although I can personally attest to all their devices that hit EOL in the years afterwards having the exact same issue.)