I think you're far away from being the only one feeling there is some arbitrary innovation curve that Apple has strayed away from. I disagree tho, because you can't blame Apple for the market and the industry, let me explain a bit.
The iPhone X was not supposed to be released when it was. Apple planned of revealing the X in 2018 and not 2017, but found themselves in a situation where they could simply not just release and get away with the 7s, or as its better known, the iPhone 8. They rushed the iPhone X through development and production to make it at least available the same year, if not the same day or month. The X was a big step, but not what it was supposed to be, that would have been the Xs, coming in two sizes, with actually better cameras and stereo recording in videos. In essence, the X was what Apple could get away with while Xs is what the X was supposed to be.
Going forward a couple of years, rumours mounted that Apple will finally introduce Pro Motion to the iPhone with the iPhone 11 (Pro), which didn't happen. Then it was rumoured the 12 Pro must come with it, but it didn't. Because Apple cheap-ed out and thought 5G and the new design would be enough for people, right? Well, turns out, maybe, but one definite reason why the 12 Pro only came with a 120hz touch sample rate instead of a 120 hz refresh rate was because Apple couldn't source the amount of displays they wanted with the specs and quality they wanted. Even if they wanted Pro Motion, they could simply not have done it. Until next year that is.
So, what happened then? Well, the X was in fact the big step Apple thought they'd make, as the general design didn't change to much from then, and how could it? How many times can you remove the home button and the chin and forehead from a smartphone design? How many times can you revert to glass on the back? How many times can you introduce more cameras? How often can you rearrange technology on the front of the phone that can't be put behind the display (as of yet)? So what can you actually expect? Because it seems like there isn't much room for anything but refinements. Even the 15 Pro is just a refined X despite Pro Motion, AOD, three cameras with one having 4x the resolution, an actual high quality OLED screen with far less inconsistency, an actually somewhat useful notch, an actually almost user friendly repairable design, a bigger screen despite a not much bigger frame, new frame materials and a new internal design to improve the felt weight, USB C, a user mappable solid state button!, dual e sim support and a SOC made for AI that wouldn't see the light of day for another one and a half years, meaning its still not out.
In my honest opinion the 15 Pro might be a contender for best iPhone in a couple of years because it is the best and most refined version of the iPhone ever, together with iOS 18 which brings features that finally make the iPhone feel mature (mainly customisation options because that's the main thing missing from iPhone since day one).
So, what can you expect from the 16? Just more refinements after the perhaps most solid iPhone release since the X ? Because what is it that was missing from the 15 Pro? IMO, nothing really. In fact, it has AI in mind, a feature we didn't even know would come to the iPhone until this year.
What's left for the 16? Well, apparently 1 button from last years Pro phone and 1 completely new button for new camera controls from this years Pro phone, a "new" camera system with last years Pro phones 48 MP camera and improved zoom capabilities including a new arrangement enabling spatial videos (yeah yeah Vision aint for everyone but its a future proofing thing), a SOC with AI in mind and overall almost the same feature set as the Pro models without the Pro Motion screen. The 16 Pro on the other hand, yeah that's more like the jump from the 11 Pro to the 12 Pro, with 5G being the camera button, but ey, what else is there to be improved, literally? Apple has no incentive to break the mould the way they did with the X, because there is no place for another X. Foldables, sure, different story. But to expect more than what we got, eh, that just shows to me how little people understand the industry, the market, innovation in technology and supply chains. Is the 16 Pro the 15 Pro S? Yes, is it just as much of an improvement as the 4s was for the 4, no, id say its considerably bigger.
Really, Apple added another button to their Pro phones and 2 to their non Pro phones and still people say they are giving up on the iPhone, I find that funny. I mean, Apple makes an obvious commitment to invest even more time and resources into the cameras on their phones, signalling their commitment to cameras going way into the future.
I think the 16 is just as exciting as the 15 Pro was and see no problem with Apples rate of introducing new features to their one ups. Having (potentially) the Rabbit R One/Humane.AI pin in your pocket, with a camera that strives to replace dedicated cameras, that's big and exciting. I acknowledge over peoples opinions on this matter, I just think they are wrong. This year is bigger technically bigger than last years but ey, wait a couple years and you see why I mean.
senseless rant over
Edit: sorry for the typos but my screen is broken and I can’t be bothered to deal with correcting anything