Was the long-standing way of referring to 75 RBIs correct or is the new style 75 RBI correct?
I think the first question that needs to be answered is what does RBI stand for? Runs Batted In or Run Batted in?
I maintain that RBI stands for Run Batted in, the singular. You would never say that a player has 1 RBIs or 1 Runs Batted In. But you would say the player had 1 RBI (Run Batted In).
That answers the question of what RBI stands for. Run Batted In. If that were not the case, and RBI stood for Runs Batted In, you could not use the singular. You wouldn’t say, Luis Arraez has 1 Runs Batted In after the season's first game. You would say Luis Arraez has 1 Run Batted In (RBI) after the first game.
With that established, it’s pretty simple. 1 dog, 2 dogs. 1 balloon, 2 balloons. 1 Bicycle, 2 bicycles. And 1 RBI, 2 RBIs, 1 RBI, 50 RBIs, 1 RBI, 75 RBIs.