If we disregard the whole 'planned obsolescence' theory, it all comes down to one integral aspect of iOS that should be of the highest priority: optimization.
Clearly iOS is only properly optimized for the iPhone 13 models and this could be fixed later on, as Apple software seems to have been coming out in a half-baked and unoptimized state nowadays and then fixed later with an update (e.g. ProMotion on 13 and MBP, webcam quality on Studio Display).
Don't misunderstand me, I am NOT condoning this kind of ****** software quality (especially coming from a vertically integrated company like Apple), but I am still positive that a fix is coming at some point.
Wish I could share your optimism!.. but I don’t think a ‘fix’ is coming.
‘Optimisation’ is what’s being used as the cover for the planned obsolescence here unfortunately, and if you look at things from a high level you could almost agree - clearly newer models have even faster hardware etc, but it you look at things at a much lower level it’s basic things that are being slowed down, such a simple opening and closing animations for apps and other UI functionality.
If you were an Apple engineer and were given the brief to make older phones ‘feel’ old and slow, and force users to upgrade, you would introduce exactly the sort of things that they’ve introduced. Ie stutter and lag navigating the UI, judder scrolling websites, intermittently unresponsive touch screen (although this only occurs when navigating, not when the keyboard is up), crackle and distortion on music if you multi task while audio or video is playing etc - yet when running a graphically intensive game there are no issues ar all…
No doubt these ‘bugs’ have resulted in lots of sales of new phones, but it’s a slippery slope as more and more people will cotton onto what’s happening, and that damages the brand.
I had been waiting for a new MBA with OLED screen to upgrade to from my old MBA (that still runs like new due to staying on macOS 10..), but I’m not interested in spending $1500 on something that gets slowed down by design, forcing an upgrade.