The other machine wasn't more upgradable, it was bigger but most importantly more powerful. You actually got more power for not that more money which is why people bought it. Or put differently: the Cube was too expensive and too slow compared to its bigger brother.
100% wrong.
You could buy a Radeon 9800 and put it in ANY G4, there was no Cube sized version. Worked fine in the Sawtooth, which was out at same time as Cube.
I found and flashed a Cube sized one but only 1 in 3 would be stable. And even then you were on the knife's edge for power. The VRM board wouldn't take it over time. For awhile there was an aftermarket VRM board that would allow you to upgrade both the CPU and the GPU.
You could put more drives in a standard G4. You could put several PCI cards in any other G4, Cube only had a single AGP2 slot. You had a much greater range of options for Optical drive for standard G4 since the Cube used a slot load versus a tray.
To refresh, you just typed "The other machine wasn't more upgradable" If you want to be right, don't argue nonsense. The Cube could only take small, less powerful GPUs. When Dual CPU upgrades came out for other G4s the Cube versions were more expensive and more trouble. If you used one with a better GPU you burned up your VRM. How does this not mean "less upgradeable"?
To get the small size they sacrificed power and compromised on heat. They originally were going to put a fan in the base of the Cube. They decided it was too noisy and left it out. So for better esthetics they sacrificed performance.
The Cube is a perfect analog to the nMP but as Apple learned, people will only go for the compromised version if you don't offer them the less compromised version.
As far as cMP being difficult to upgrade. YES< YOU HAVE TO DO RESEARCH. But guess what, the PCIE SSD I bought works GREAT as a boot drive. Wanna know where it won't work? In a WINDOWS machine. Some people just can't handle small fiddly stuff. Apple demanded that you drag your entire 50+lb Mac Pro into a store if you wanted a 2oz WiFi card installed. It was NOT considered "user installable" yet MOST people could do it just fine.
Moral of story, some people just need to admit that this kind of stuff is beyond them and have a Pro do upgrades for them. But that doesn't explain removing the option altogether.
Anyone who has taken a Cube apart knows how much it has in common with nMP. Compromise after compromise. Makes sense today, will seem hopelessly limited in a year or two when nnMP comes out with Nvidia Titan Z or Hawaii GPUs or new CPUs on new socket.