I think they keep going with Rock for the same reason that they keep the Fire/Water/Grass trio: it's simple. For example, if they made a Rock/Flying/Fighting trio, not all kids who play the game would automatically know that Flying is Super-Effective against Fighting. It makes sense, it's just not that obvious.
Water putting out Fire, and Fire burning Grass is obvious. (Grass > Water may not be as obvious, but the point stands.)
Rock is obvious; it's something most kids either encounter daily (cement) or at least know about. They learn in school how over time, water can erode even the toughest of rocks, and they've seen that trees' roots can break concrete.
As much as I agree that it would be nice to have a new type as the first gym, I wouldn't be surprised if the 5th gen had us fighting a Rock Gym first.
Water putting out Fire, and Fire burning Grass is obvious. (Grass > Water may not be as obvious, but the point stands.)
Rock is obvious; it's something most kids either encounter daily (cement) or at least know about. They learn in school how over time, water can erode even the toughest of rocks, and they've seen that trees' roots can break concrete.
As much as I agree that it would be nice to have a new type as the first gym, I wouldn't be surprised if the 5th gen had us fighting a Rock Gym first.