, Apple would have to sell a lot of upgrades to make it worth their while .
Develop that, I actually dont undestad your point.
Hence an open design - including some aspects of OSX - might or might not lose them a few bucks at the initial purchase, but gain them a lot more in units sold and market share .
Disagree, Apple is used to "train" their customers to buy their products the way they sell, Apple's premium in propietary hardware pays itself, Guys from PC world used to DIY upgrades may see it inconvenient but Fanboys (and corporate) customers like this way, I come from PC (ok, first mac, the pc, and now Mac again) initially was my thinking, to purchase bare minimal configurations and then upgrade as I need, I do that on my first MBP(ram+storage+wifi/bt), and the MP(ram), Mini (storage), But also I'm aware DIY upgrade sometimes are risky(I burned logig boards upgrading GPUs), even time consuming (compatibility issues), nothing this happens if you just buy original sys->sell outdated original sys->buy new better spcd system.
A game changing Approach would be a Cartridge-like GPU solution, one with Apple's blessed compatibility and high quality std, plus simple KISS install, one that do not need you to spend time solving issues, or risk to system damage.
I read very carefully Apple's signals about the mMP, and points a Modular system (segregated display at least), upgradeable (At BTO or after market ?), and ALL NEW proudly Macintosh (which means innovative, groundbreaking, beautiful).
More strong rumor point Apple working with AMD on a Full AMD based MAC, maybe Apple delayed the mMP release to adopt the Epyc cpus (8/16/24/32 cores) and TB3 to be free from Intel (intel released TB3 to public domain on Jan 1), OK maybe the MP to be Xeon based, and this AMD cpu (an APU too) goes to the next Mac Mini (VR capable).
Personally I doubt Apple to resign the Mac Pro peripheral control (allow DIY PCIe upgrades), It means Support Nightmares (very expensive to apple), more work on macOS compatibility (as 3rd party GPU vendors will require support for its drivers), a long ETC that doesn't profits to Apple.
The GPU Cartdrige (or something alike) is an very strong possibility on the Next mMP, but I wont be surprised, if Apple just re-launches the Trashcan just grown enoutg to host dual 350W GPU plus a 32core unique CPU, Upgradable only at Build time, and maybe requiring authorized service just to upgrade RAM (as the iMac Pro), mathematically it will require an about 37% bigger trash can, or something alike.
I was very surprised how Apple didn't include a little window to allow DIY ram upgrades on the iMac Pro, and it actually is an not a good signal for the Mac Pro 7,1.