Apple under Jobs had an aggressive iPhone update schedule which just isn't present under Cook, and this make many iPhone fans unhappy, imo.
After the original iPhone, there was a product cycle of redesign one year and feature heavy update the next. I think this started to change beginning with the iPhone 6s, as its biggest feature turned out to be kind of a flop (3D Touch).
2nd Gen Design:
3G - Redesign
3GS - Speed! many internal upgrades with a focus on performance, along with improved camera, which becomes standard for every "s" model year
3rd Gen Design:
4 - Redesign (the most beautiful and unique design, imo)
4s - Siri and Retina Display!
4th Gen Design:
5 - Redesign
5s - Security! Touch ID was a big feature for this year
5th Gen Design:
6 - Redesign
6s - 3D Touch?! First year that the big feature wasn't really that big
7 - No Redesign! Hardly an "s" model upgrade. Minor improves that are expected, and the best upgrades are on the Plus size only, Oh, and losing some key features such as headphone jack
8 - Wireless charging, glass back! The real "s" model of the iPhone 6, just two years late. The aggregate of new features of the 6s, 7, and 8 should have been the 6s.
6th Gen Design:
X - Redesign
XS - Upgrades that one would expect, but no big feature
XR - A new model?? Kinda of new, but basically the X without OLED display
11 - The "s" of XR, without a big feature
11 Pro - basically the 3rd generation of the X, with the camera being the big feature, but something that was expected in the past.
To sum up, the iPhone redesign and "s" cycle upgrade was messed up with the iPhone 6 design and could really be seen with the iPhone 7 launch. This was around when the excitement left the forums and the new iPhone articles in the news no longer made front page.
Some of that came back with the X, but going into the third year with what is basically an upgraded iPhone X, people are disappointed, or just don't care anymore.