Actually, continually providing upgrades to older iPhones is a wonderful way to push new hardware sales: the upgrades significantly degrade the user experience because the hardware isn't as capable.
I wasn't implying they are introducing bugs intentionally. In the normal course of adding capability to new releases of an operating system tailored to newer phones with better hardware, it's only natural that older phones will run those new operating systems slower and the experience will degrade. Nothing conspiratorial about it. But, it works really well for Apple, because they can charge you to upgrade to the new iOS version, and then in 6 months when you're sick of your laggy, unresponsive old-ass iPhone, they can helpfully sell you a new one.
I'm sure others haven't been so lucky.
The idea that Apple is purposely introducing bugs to degrade user experience is silly. There is no conspiracy.