I like it, but it's not proprietary enough for Apple to make. They didn't come out with a single upgrade GPU for the Mac Pro dustbin design, so I doubt they care much about selling modularity or upgradability these days. Their message is clear: "upgrade" means new device, which means all the products they sell are disposable appliances.
IMO, they could open up MacOS to work on some variations of hardware and sell it for $300-$500. They would still make the nicest hardware, but since desktops and laptops obviously aren't their main focus anymore, they could still offer power users good tools to build all these amazing mobile apps on without limiting them to their current slim pickings. Hell, even just make XCode cross-platform. Even just let it run on X, and use the Windows Linux subsystem to support it.