I know there have been cases where companies seem to intentionally slow down old phones to encourage new sales, but it doesn't really even require an "evil" motive. By releasing new hardware yearly, they are dramatically increasing their workload by having to support every device. On top of that, there's the perverse incentive that spending the money to release timely, high-quality updates to previous-generation devices will actually have a negative impact on their bottom line by reducing new sales.