SpaceMouse has a Z-axis (lifting the knob up), which is really useful to zoom in and out of a 3d model. That can't be captured with just IMUs.
What if you count rotation around the vertical axis? I have a kensington trackball where that is how you scroll.
Then you can no longer rotate, unless you make it rotate when you push some additional button. The 3DConnexion devices have 6 degrees of freedom without pushing any additional buttons. That being said I have a SpaceNavigator and I don’t use it, it’s just sitting in a drawer and I use my regular mouse and keyboard even in Blender.
Makes me wonder if it would make sense to have rotary encoders for precision work -- something that would work like in an Etch A Sketch. The pointer could be positionned with great accuracy and wouldn't move by accident.
It may not be too hard to make a prototype with an Arduino. Or maybe somebody already tried it?