Why completed modernisation requires a later war

How would the combat fleets change if the major navies completed their pre-war modernisation plans before World War II?

A delay of roughly five years produces larger, more modern fleets. They also differ from the original plans because every navy reacts to the others' construction and technology.

Granting every navy the ships in its 1939 plan does not produce a stable alternate history. Roughly 1944 provides a workable comparison point, giving the major powers time to advance their construction programmes. Those extra peacetime years also change the aircraft, ship designs, budgets, and strategic assumptions behind the original plans. Each navy would respond to new information about its rivals, so its later orders cannot be predicted precisely.