Annual development is less per product. However, I don’t completely agree with you. 3 of the last 4 iPhone flagships were pretty big upgrades compared to their predecessors.
The iPhone X brought a new futuristic design with the home button gone, the addition of Face ID, a much better screen, etc. – in my opinion the biggest change of all iPhones ever launched.
Then you had the iPhone XS, which I think is probably the smallest upgrade ever compared to its predecessor because you have a bit of news – the big new thing was the new size with the iPhone XS Max. So about this upgrade, I agree it’s not worth upgrading from the iPhone X. But it was another huge step forward from any other iPhone.
Then I got the iPhone 11 Pro with the addition of another camera, much better lenses, a much better front camera, a brighter screen (a task outdoors), a much higher contrast screen, a faster slide, and a new rear.
Then you got the iPhone 12 Pro with a new design, better cameras, 5G, and MagSafe. I found it hard to resist upgrading from 11 Pro to 12 Pro compared to upgrading from iPhone X to iPhone XS because of the new design that looks great.
Again, I think evolution is stagnant all over, but that’s normal. But on the other hand, I find that the intergenerational improvements in the last few iPhones are greater than before with iPhones and also greater compared to the upgrades in competitors.