What Admiral Jellicoe feared was that the Germans would modify their existing battleships, such as the Koenig class, with 17in (43cm) guns. I have seen the possibility belittled in some published sources, but I think that if Germany had not been so focused on the losing proposition of using submarines against merchant shipping, they might have found the resources to build the 43cm guns and to modify their existing ships.
Using submarines against merchant shipping was really mistaken in WWI. Perhaps the main reason was political, rather than strategic, in that unrestricted submarine warfare brought the United States into the war. That was the real reason that the Allies won. Without that intervention, the war might have ended as a stalemate, with Imperial Germany surviving. What Germany needed to accomplish at sea was to break the British blockade. That was not going to happen by attacking British merchant shipping.