We tried a similar rule on our One Piece forum. It forced people to pick their strongest argument instead of listing every single point. Debates about the Wano arc got much sharper because folks had to choose between talking about the art or the pacing, not both at once. The quality of replies went up when the opening posts were tighter.
On the My Hero Academia threads, we set a word limit for initial posts to keep things focused. It helped cut down on rambling and made debates more pointed. Now people get straight to the good stuff.