Lets suppose you use caravans in the early ages, and once you go to industrial age (or discover railroad or what ever), caravans are made obsolete, and trade moves to a streamlined diplomacy option.
I love that one. It actually reminds me of Colonization: at the beginning of the game, you had to move your small merchant fleet from one place to another, and it was fun. But at one point of the game (after completing a "Wonder", i.e. a founding father), you could build a city improvement that took care of transatlantic trade all by itself. As such, trade fleet was fun at the beginning, but didn't groy boring at the end