I've owned an X, a 12, a 13 (after the 12 got water damage), and have ordered the 15 (but not received it yet).
With the caveat of not having used the 14 or 15 (but just looking at the specs on paper), I would have to agree with the comment you're responding to that all the changes since the X have essentially been incremental. Not only that, completely and consistently annoying UI bugs such as the one where you accidentally tap a notification when you intended to tap a UI element at the top of the screen (which I blogged about way back when: https://medium.com/p/31773fe6bbd5) have not been fixed despite the fact that my solution (adding a small input "debounce" or timeout whenever the UI changes, where inputs are ignored) would likely solve this problem. (This problem has likely annoyed YOU, even if you haven't heard about it until now! More generally, it's also a problem with websites that let you accidentally click or fat-finger on elements while they're still re-rendering and shuffling around, which my post also includes.)
Of course, that's a software problem... The hardware is largely exactly the same. There's still no way to TouchID through the screen (like Android has had for years... I absolutely cannot stand FaceID and just use an alphanumeric code to unlock with a long unlock time now), there's still no models with a foldable screen (again, like Android has had for years), and we are only JUST getting USB-C (whoop de frickin' doo... again, not an Android fan but Android has had this for years too) and ONLY (more or less) because the EU twisted Apple's arm.
(Aside: I still can't directly edit the custom spelling dictionary iOS maintains internally. Mine has a ton of garbage in it, but is just useful enough that I don't want to wipe it completely, which is the only option.)
And now with iPhones dominating the teen market with like an 88% marketshare (which is a fairly good indicator that Apple will be completely entrenched for years from now), they just don't have any incentive to take innovation risks anymore.