Waiting until 1948 would not solve Plan Z's strategic and economic contradictions

Could Germany simply wait until 1948, finish Plan Z, and then decide whether war was winnable?

No; continental threats, economic weakness, and accelerating Anglo-American construction would make the later balance even less favorable.

The counterfactual cannot isolate naval construction from Poland, the Soviet Union, or Germany's worsening finances. Resources assigned to Plan Z would still compete with the army and other continental requirements as neighboring states rearmed. Britain and the United States could also continue expanding in peacetime, when construction was cheaper for them than emergency wartime building. Economic desperation did not amount to a settled intention to start war in 1939: Hitler kept gambling that territorial gains could sustain the regime without provoking the conflict that followed.