Just because an idea wasn't implemented or didn't become popular doesn't mean it's not a good idea. I have always strongly felt that expansion cards in desktop computers should work more like this rather than requiring the case to come off.
NIM modules and CAMAC modules have been popular instrumentation platforms for experimental physics since the 70's and 80's. Of course that's very far from consumer adoption but the concept kinda works. No idea if NIM modules are still in production though!
https://en.wikipedia.org/wiki/Nuclear_Instrumentation_Module
One of the many problems with sidecars is that they make the computer footprint larger. While it hypothetically allows for unlimited expansion compared to a limited number of internal slots, this would be practically limited by physical space and electrical signaling issues.