Well, I'm speaking more in relation to how a lot of it's features have been made deeper in later entries. So when you look back, IV has a lot of systems but the mechanics of each is shallower. In VI systems are richer but you have a bit less off them. In IV, you as the player make less meaningful decisions and less interesting variation unfolds from game to game. Systems that basically give you rote chores to make optimal choices gets old pretty fast. Outside if exploration, choosing where to settle, and your basic victory path, the rest played itself. Stacked units and workers were always such a boring part of the game to me. Civ V sort of inverted the issue by having one unit per tile, which made open combat more interesting but frustrated siege efforts. Also just working the map with workers is always a really boring exercise in chores. So Civ VI in my mind really fixed a lot of the foundational issues in creating interesting map play, and breaking from the more optimal build strategies that dominated IV and V.