Their current business model incentivizes Apple to make high quality products that last a long time. The larger Apple's installed base is, the more money they make from the App Store. You can tell from the used market prices for iPhones that they've been doing a great job of this.

This also happens to be more environmentally sustainable, so it's not in society's broader interest for Apple to change their business model.

A world in which Apple only earns profits from hardware sales removes those incentives. Not only would they try to get you to upgrade more often, they'd also invest less R&D in the hardware as well. Instead of doing the iPhone X redesign, with its higher component costs, they'd try to milk the iPhone 6 design for as long as possible.