I do agree that there is a point where there is no point to expanding any more. But expanding remains viable almost until the end of the game because trade routes mean a new city gives decent returns right off the bat. Also, each new city is only +5% increase to base tech cost, it doesn't take much to be worth that small penalty. Out of the 12 virtues not on the way to the free colonist, only 3 affect health. A big theme of prosperity is health but it isn't ALL about staying healthy, or even mostly about staying healthy.