Well, in fact that is kind of how I see it. 1) Computers designed to sit on the floor and plug a monitor into, 2) computers designed to sit on a desk and plug a monitor into, and 3) computers designed to sit on a desk with a built-in monitor. In my experience, I've never really come across any other type. Now granted within each major form factor, there all all kinds of configurations and functionality, but at the end of the day, I only see three major form factors.
Granted. I had almost forgotten about the value of video cards in those days. I don't think most of the LCs could really use the video capture cards due to their processor power, and a stereo sound card was not really very useful in those early days. But you're right. My point is, by the time the Cube came along, and especially the mini, all of the things one would use the single LC expansion slot for were already standard, or better served via FireWire devices (in fact FireWire is a perfect example of superiority for multiple functionality over having even one, or two slots for those things). Video is about the only thing a person might want to upgrade on a consumer desktop, and that's mainly for use by hard-core gamers and graphics professionals ... Something the original LC pizza-box series would have never been used for in the first place. Let's face it, the standard video in most computers produced in the last decade is overkill for most end users, unlike the original LC, which only supported 4-bit video at 640x480. So in that sense, the Cube and mini are obvious successors in my mind to Apple's otherwise extinct form factor ... That is unless the LC pizza box is not considered a desktop in the same family form factor as the Mac II series.
Are the Cube and mini superior to the last G3 desktop (processor speed aside)? No, but then neither was the Classic II superior to the SE/30, yet no one has ever challenged that it was not the successor to that form factor.