> you'll sell units
Not just sell units, and probably not sell (many) more units, but you can sell units at a way higher profit margin. As pretty as the new Mac Pro is and as amazing as it is as a piece of technology, I have to believe that the one-time R&D costs of the Mac Pro are huge and directly contribute to the stagnated pace of upgrades.
While the cheese-grater Mac Pros weren't nearly as slick-looking as the oversized coffee cans, Apple could effectively throw out the whole internals of the cheese graters every year or two, put a mostly generic motherboard in it, with a wholly generic GPU and completely vanilla CPU/Memory/Disks and have a whoppingly fast machine with relatively minimal R&D cost, even while upgrading it in lock-step with external (intel/Nvidia/etc) evolutions. With the coffee cans, every single upgrade will require a huge redesign at a huge expense due to the tight integration.
While I understand the benefits of and necessity of tight integration in basically every single other product that Apple sells, its antithetical to the point of a Mac Pro.