You are comparing apples and oranges.The US Navy began the War in the Pacific with 4 operational aircraft carriers, four battleships, and a significantly reduced fleet. By the end of World War II, just three years later, the US had nearly 7,000 operational warships, including 28 aircraft carriers and 23 battleships.
The industrial capabilities of The US and the ability to quickly produce weapons is more that able to keep up with the increased requirements should we find ourselves once more in a global war.
Today's weapons systems are INCREDIBLY more complex than they were in WW2.
The premier class of ship for the US Navy in WW2 was the Essex class, aircraft carrier.
It took only about 2-2.5 years to build.
The Nimitz-class Aircraft carriers took over 6 years to build.
The new Ford-class - at best - takes 6-8 years.
Heck, even an Arleigh Burke-class Destroyer takes 4-5 years now to complete.
Even attack submarines (the new Virginia class) takes 4-5 years to make.
Today - in terms of major weapons systems - you go to major wars with what you have in service and in reserve/storage.
And that is pretty much it.