Here’s another angle:
An iPhone 4 with the “last supported OS” is significantly slower than when it had the OS it launched with.
This is true across the board, which leads to this very convenient “they can buy used iPhones, but we don’t want them to use them, so we’ll gently guide them to new devices by making it inconvenient and annoying”
And before someone argues: Yes, much of the slowdown is because of new features, BUT Apple could simply allow users to disable most of them (such as ML-processing photos in the background, or deep-indexing file contents for spotlight. Those are not required for the phone to function as a phone).
If someone wanted to use the iPhone 3GS with the original iOS, not sync to the cloud, replace the battery every couple years, and install a firewall to prevent intrusion via known attack vectors, they could realistically have a perfectly snappy and solid experience in any future decade. Doubly so if they kept a 2.4Ghz AP when the industry is on some unknown future frequency.
However: Apple has taken many steps to prevent exactly that sort of thing, and they will continue to make as many as they can get away with.