Digic 6, 6+ et al are just specialized versions of digital signal processors, a subset of microprocessors.
Each one builds on the previous version, using redesigned architecture, process shrinking (smaller chips run faster as signals don't travel as far) and other techniques. Some cameras add more processors rather than a more expensive one.
At some point, you have to "freeze" the design process and start making them.
What happens is the older designs get used by cameras further down the food chain or simply dropped all together.
There is usually "headroom" in a chip where you can squeeze better performance out of it by making the firmware more efficient, like the most recent upgrade to the 7D.
As the chip process becomes more mature, they get better "yields", as a typical start can be as low as 30% and get as high as 70% of the specification they want.
Some processors have higher tolerances for clock rate, so they may become "Digic 6+" rather than a mere "6". You can't simply upgrade a cameras chip as these become readily available as you may need to tweak or redesign other parts of the camera