Being able to ski in control of speed and line, with rythm, on black/ double black terrain is a big milestone.
To draw a video game analogy, once you get to that "max level", the true end game progression begins. With numerous "paragon levels" and achievements to unlock for surface snow conditions, powder, ice, all the flavors of crud, bumps, trees, chutes, steeps, gates, park, etc... These unlock greater efficiency, the ability to ski all day long for days in a row, increased enjoyment and appreciation for bumps and crud, and compliments from other skiers like, "he's not bad", "he's good for a dad with three little kids", and "your smooth".
Expertise also implies knowledge of the sport, reading terrain, care for your gear (basics of tuning), selecting proper gear (boots and skis).