Most systems on chip involve a combination of analog and digital circuitry. While together they may be considered mixed signal, the discipline of mixed signal engineering involves understanding both worlds and how to integrate them effectively. Typically this involves a combination of an analog design, a physical layer (PHY), and an analog-to-digital or digital-to-analog controller.
While this is an excellent way to optimize a design for power and performance, the challenge has always been getting engineers to embrace both disciplines. This has proved extremely difficult over the past couple of decades, to the point where most EDA companies have decided that their efforts would be better spent trying to provide some sort of translation or bridge between analog and digital designers rather than waiting for the industry to adopt mixed signal engineering.
Both analog and digital will remain important in chip design. Analog is the bridge to the physical world, but it does not scale effectively for each new process node. Digital, in contrast, scales from a performance and power perspective in accordance with Moore's Law and Koomey's Law, respectively.