Apple does a crazy amount of work to keep iOS running on as many devices as they can. Speaking as a developer, I wish they'd be more aggressive about dropping support for older devices.

But even if the new OS runs slower on old devices, that's not even remotely Planned Obsolescence. Planned Obsolescence means they deliberately make older devices run badly in order to drive upgrades. But there's not even a snowflake's chance in hell that Apple is doing that, and saying they are is pretty insulting to all the work they do ensuring the OS is at least usable on those devices. The simple fact is, as devices get more powerful, the OS expands to take advantage of that power, and so when you run the OS on older hardware it's now doing more work than it used to and frequently ends up slower.