Customers don't, but Apple does because it drives the hardware upgrade cycle.

Also, employees do because it drives the promotion cycle.

Does it really? When systems get 5+ years of supported OS upgrades, it doesn't seem like the annual OS release cycle is driving users to upgrade hardware in any meaningful way. If anything, the hardware update cycle is driving the software update schedule, because there needs to be an OS to run on the new hardware when it ships. This is especially apparent for the iPhone.