Mobile devices are typically underpowered compared to desktop systems. They've caught up to PCs in terms of power (as evidenced by the iPad Pro hardware being used as the basis for the new dev kit by Apple). But the early years, in particular, saw massive annual increases in performance, Moore's Law had stalled on PCs but was alive and well for mobile platforms at that time.

You have to track the actual hardware changes in that time and compare it more to the PC market.

PCs pre-2006 had almost universally 32-bit processors, fixed pipeline GPUs, and maybe 4GB of RAM, probably 2GB. Post 2006, more GPUs got hardware shaders, 4GB of RAM became the baseline, as did 64-bit CPUs (usually with 2 or more cores). Those have all aged reasonably well and can still run most modern software outside of the graphics intensive ones (and even those sometimes if you turn most of the settings down).

The iPhone 3GS (2009) was the first one to have hardware shaders. That right there knocks out the two prior generations from being able to keep up.

The iPhone 5S (2013) was the first 64-bit processor, creating another boundary line for long-term support.

The iPhone 6S (2015) was the first to have > 1GB of RAM (upped it to 2GB), which created another boundary line.

So it's not too surprising that the higher RAM, 64-bit processor, with hardware shader support is the oldest supported iPhone (from this year forward).

Post-2015, the iPhone has improved (obviously if you've switched from a 6S to the 11 like I did) in performance and capabilities, but it's also stabilized in many ways. I haven't dug into the post-2015 phones yet to see if there are any other natural boundaries that will show up as devices drop out over the next 2-3 years, but I'm sure they exist there somewhere. Eventually the OS and the applications will be dependent on specific hardware features that cannot be easily (or efficiently) recreated on old, lower clocked processors.