What makes a delayed-war counterfactual credible?

Has a delayed Second World War already been exhausted as an alternate-history subject?

There are many treatments, but credible versions must respect construction time, industrial capacity, and the absence of wartime production programmes.

A three-year delay cannot produce fifty British battleships or a completed Plan Z. Large carrier programmes also need a production base capable of delivering them. Shipbuilding depends on yards, labour, machinery, budgets, and designs mature enough to order. Wartime emergency measures cannot simply be assigned their historical dates after the crisis that prompted them has moved. A credible counterfactual begins with programmes already underway and follows their plausible continuation.