Interesting definition of a desktop, having at a minimum a single expansion slot. I draw this conclusion based on the Mac LC, a desktop, which only had one LC PDS slot, and was otherwise not expandable. Now my argument here would be, that the one expansion slot on an LC would have been used most likely for Ethernet, as were most PDS slots in these entry level Macs, since most were not powerful enough to really drive some of the dedicated application cards available, or large enough to accept them. In which case, Ethernet is a built-in feature of the Cube and the mini. And arguably, the potential to expand is far greater in a slotless USB, Firewire, and now especially Thunderbolt equipped Mac, than in the last G3 model.
That said, it would seem logical that a desktop then is more defined by its form factor, than ability to expand. Nevertheless, a desktop would seem to be defined as a family by the ability to place the computer on top of one's desk (in part to easily access CD drives and ports), and requires an external monitor. In this case, even though the Cube is not a direct successor to the G3 desktop in terms of function, it is clearly the only successor to the "desktop" family form factor. Likewise, the Mac mini, is then the logical successor to the Cube, even if it was a less powerful computer. I would argue this for the same reason that the LC, LC II and LC III are in the same family form factor as the Power Mac 4400, Quadrant 610, etc., despite having few of the same functional, or expansion abilities. And even though the mini is a less capable mac than the G3, or cube, no one disputes the Mac Classic II as the successor to the SE/30. Additionally, towers all have specific expansion abilities typically not found in any desktop. In the case of the Cube, it is clearly not intended to be placed on a floor given it's need for ventilation and access to cables and disc drive, and likewise, the mini would be difficult to access on the floor level given it's profile. Can you just see someone down on their hands and knees trying to swap discs? Since they are clearly not all-in-one designs, that must mean they are part of the desktop family – perhaps a new branch off the main branch on which the G3 marked the extinction of that species.
That said, it would seem logical that a desktop then is more defined by its form factor, than ability to expand. Nevertheless, a desktop would seem to be defined as a family by the ability to place the computer on top of one's desk (in part to easily access CD drives and ports), and requires an external monitor. In this case, even though the Cube is not a direct successor to the G3 desktop in terms of function, it is clearly the only successor to the "desktop" family form factor. Likewise, the Mac mini, is then the logical successor to the Cube, even if it was a less powerful computer. I would argue this for the same reason that the LC, LC II and LC III are in the same family form factor as the Power Mac 4400, Quadrant 610, etc., despite having few of the same functional, or expansion abilities. And even though the mini is a less capable mac than the G3, or cube, no one disputes the Mac Classic II as the successor to the SE/30. Additionally, towers all have specific expansion abilities typically not found in any desktop. In the case of the Cube, it is clearly not intended to be placed on a floor given it's need for ventilation and access to cables and disc drive, and likewise, the mini would be difficult to access on the floor level given it's profile. Can you just see someone down on their hands and knees trying to swap discs? Since they are clearly not all-in-one designs, that must mean they are part of the desktop family – perhaps a new branch off the main branch on which the G3 marked the extinction of that species.