Why Plan Z needed Britain to stand still

What was fundamentally wrong with Germany's Plan Z?

It assumed that rival navies would not respond, although Germany lacked the maritime infrastructure and resources for a prolonged naval race.

German naval freedom depended on exceptionally favourable early victories in Norway and France. Without those outcomes, surface ships would be boxed into the Baltic and North Sea while Germany lacked enough submarines or escorts to change the war at sea. A delayed 1948 confrontation would not preserve the 1939 Royal Navy in amber: Britain would have more carriers, advanced destroyers, radar, and perhaps jet aircraft. German yards and finances were already strained, while British industry retained far greater capacity. Plan Z treated a moving strategic competition as if only Germany were allowed to build and adapt.