How is what you're describing not planned obsolescence? The original iPhone had CPU and RAM* capable of powering the most common app usages of modern iPhones. Why should we need way more resources to do the same tasks as before?
*Not including GPU here cause it's a different story.
You're ignoring the fact that some of those resources are used for background tasks and other pieces of the OS. Just because an app doesn't require more resources from one OS version to the next doesn't mean the system as a whole doesn't, also eating into the limited pool available.
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Midrange and budget phones may not get updated, but you get what you pay for afterall. All android flagship phones will get at least 2 years if OS updates and 3 years ot security updates.
Just 2 years of OS updates is pretty meek considering you're buying a flagship. Apple provides anywhere from 2-5 for all models, even the most basic.