It doesn't really feel like a significant opportunity cost to me because the outermost hexes tend to be unworked by the city unless its something awesome enough to assign a citizen there. They generally will be empty until the city has the population to work on third ring. To build it, it simply means there's a pretty tough enemy that concerns China enough to build a great wall to keep trump out and make him pay for the wall . And then throughout the ages, the need for a wall in a specific hex will generally fade as you want to build something else in it's place. Of course this is all speculation, I don't know how quickly a population can grow to cultural growth in Civ6. If precedent holds, Culture grows quicker than population unless you feed the city hardcore. Something which I only can find out when the game comes out.