> Moore's Law had stalled on PCs but was alive and well for mobile platforms at that time.
That's not a very convincing argument. Moore's law was alive and well two decades ago for PCs, but Windows XP (initially released 2001, supported til 2014) was able to run on hardware from 1995 (according to https://www.winhistory.de/more/386/xpmini_en.htm).
That gets into a difference in motivation as well, but watching hardware changes (for iOS) explains a lot about why different phones have lost support over the years.
Microsoft has a different motivation, in that they were selling Windows and Windows support. Apple didn't sell iOS (and they'd stopped selling OS X sometime in the '00s). MS had a vested interest, then, in supporting all hardware that they could because of businesses (and governments, especially governments) that wanted the latest OS, but didn't want to drop $2-3k on new computers. MS made their money off of the OS, their software on that OS, and the money spent by developers to have access to MS's development tools.
Apple makes their money off of their hardware. So some people like to claim that Apple does a shit job of supporting old hardware, which may be true. They tend to cut off support more readily than many people would like. But in mobile they do a ton better than any Android device manufacturer (who have the same motivation as Apple: selling hardware).
But my point still stands, older versions of iPhones are harder to continue supporting because they lacked hardware capabilities/features of newer ones. From improvements to the GPU, increasing RAM, adding cores, and going to 64-bit. Each of those added capabilities that the OS and app developers took advantage of that created boundary lines that barred older devices. This is more akin to the changes from x86 to x86-64 processors than the 1995->2003 hardware changes (Pentium 1 -> Pentium 4, all 32-bit) and the change from fixed-function GPUs to hardware shader GPUs.
That's not a very convincing argument. Moore's law was alive and well two decades ago for PCs, but Windows XP (initially released 2001, supported til 2014) was able to run on hardware from 1995 (according to https://www.winhistory.de/more/386/xpmini_en.htm).