A motivation for no longer reporting unit sales was that analysts paid way too much attention to those numbers. I recall several quarters where device sales fell just a little and people would proclaim the soon death of the company[1]. Though unit sales can be inferred from revenue, as the company moves into services it benefits them to group all device sales under one line item because it's a business category that doesn't have the crazy growth it used to.
The Apple Watch is already a larger revenue generator than the iPod ever was. AirPods are either thought to be larger or soon will be.