For me the issue is the way the add-ons were engineered and then marketed. There was some hype for some, but mostly they were sold as expansion formats for the base console or peripherals. The 32X was stripped down from its original design strictly because Sega of Japan saw it as a peripheral. That is, no different from the Xbox 360 Kinect or Playstation Move.
We might like to think of these add-ons as total system upgrades, but most of them just weren't. If we want to look at it otherwise, we had better start looking at NES and SNES chipped carts as separate systems, along with Saturn and N64 RAM expanded games. It seems silly at that level, but that is actually how silly it is with the Turbo CD and Sega CD as well. Even the 32X is basically just an add-on that overlays graphics and audio over what the Genesis could already do, without the Genesis it would be even more silly than popular nonsense makes it out to be.