The same idea extended to the SNES too if I recall, a quick dig up on Wikipedia:
"The system was designed to accommodate the ongoing development of a variety of enhancement chips integrated in game cartridges to be competitive into the next generation."
We have some sorts of similar things. EyeToy and Kinect were additional hardware added to the system for games; not on the same media as the games though. The difference is expansion hardware today is always going to be subordinate to the main system --- the economics and data transfer realities don't work out to put a polygon processing enhancement in a USB add-on; but the cartridge slot has immense potential and it could also be cheaper to use the base system with enhancements than to release a whole new enhanced system (although, it's not clear to me how much the base genesis/megadrive benefits the 32x)
Well with xCloud coming up from Microsoft, and Sony partnering with them on cloud gaming - we appear to be going in the opposite direction, for better or worse.
"The system was designed to accommodate the ongoing development of a variety of enhancement chips integrated in game cartridges to be competitive into the next generation."