Is that not the pattern Apple has followed since the release of the original iPhone? This is an "off" year, the device would have been called the 12S but they stopped that pattern a while back.
This is a mischaracterization of the S models. Even though the non-S models had bigger design changes, the S models were often large technological leaps:
- 4S: single -> dual core
- 5S: first 64-bit iPhone, first Touch ID model
- 6S: twice as much memory, 70% faster CPU than the 6. In contrast to the 6 still supported, probably because it has so much headroom. Also 3D Touch (though it ultimately failed).
If anything, it's the screen tech that could be holding them back, there's supply constraints in that chain as well.
I'm genuinely suprised we even got variable refresh rate; Apple clearly had much higher standards for it or they were concerned they couldn't meet demand, otherwise it should have shipped 2 years ago.