Earlier quoted context omitted.
I thoroughly doubt that Apple's endgame here is to make their users the product – given that the operating system is only available to people who have paid them money in the first place. A much more pragmatic conclusion is that they think this feature is something that users want. And in Apple's case, as well as others, it probably is.
You paid for the hardware and the OS that shipped with it. Developing a new OS version costs Apple non trivial amounts of money - development, deployment, testing, bandwidth, support etc. and they used to charge for them up until just a few years ago. Something's got to fund it - things like pushing more of Apple's online services, gathering user data etc provide some ROI to Apple.
They innovate in the hardware space and physical design space, but all of their competitors will make copies of their hardware advantages after a few years. To keep on being competitive, their entire experience has to be superior. And what is harder to copy properly is the software experience.
Engineering cost wise, it's cheaper to just keep the software train rolling than to have older versions persisting that you have to continuously support. That is why they don't charge for it, because it's cheaper for them if you upgrade sooner and become part of the marginal few they can ignore if your a straggler. And 'free' has far higher adoption rates than even $1.00.