Also, on many fronts Apple drew the line in the sand years ago with 64-bit support -- for iOS, apps, you name it. The A7 was the first 64-bit Apple SoC and the platform transition to 64-bit was complete less than two years later. That's why the nearly 8-year old iPhone 5s is still supported, while the 32-bit iPhone 5 was EOL'd with iOS 10 (security support ended in July 2017, with only a fix for the GPS clock bug issued since then).Can't be upgraded due to hardware requirements (mainly RAM)
As to why Apple continues to support the older 64-bit devices running iOS 12, it's simple numbers. The iPhone 6/6 Plus remains the biggest selling iPhone generation ever by a wide margin. Even if the majority of them are no longer in daily use, there are still likely tens of millions of them that are. My iPhone 5s was my daily driver from September 2013 until a year ago when I upgraded to the iPhone SE 2020 (I still prefer the smaller form factor on the 5s).
Platform security is one of Apple's big marketing points, so minimizing the number of showstopping security issues is one way of making sure that current customers remain future customers.
But, Apple is also transitioning more into a services company. Apple no longer breaks out the unit sales for their iOS devices, but the estimates I've seen indicate that iPhone unit sales have flatlined or declined. Only with the introduction of higher device tiers with higher average price points has Apple been able to maintain device revenue growth.
Apple's biggest growth though has been with services. A lot of it comes from the App Store, as well as newer launches like Apple TV+ and Apple Music. And the best way of ensuring revenue growth with services is to ensure that as many devices as possible can support those services. And security is a big part of that.
When Apple abandoned iTunes support for OS X Lion, they also cut off support for iTunes Store purchases because our iMac had 32-bit firmware. I could still use the application, but could no longer make any transactions because the platform security now required a 64-bit UEFI.
Since iOS 12.5.4 covers WebKit, I would assume that this ties into Apple's services and their security as well.
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