Horse pucky.
U.S. industrial output is greater now than the 1940’s.
Too much bureaucracy, too much in the way of environmental nonsense, and the ever-present gold-plating of weapons systems. How much of our raw materials and a wide variety of sub-assemblies come from overseas - quite a lot from a country it would be necessary to have a WW2-type mobil\zation against (China) to defeat?
Note also that the easy-to-mine, high-quality, iron in the Mesabi range in Minnesota are largely mined out - now we have to use taconite, which requires far more processing (which means added time and expense) to get the same quality of steel. https://en.wikipedia.org/wiki/Mesabi_Range
The P-51 went from an idea in a signed contract to a prototype in 102 days, and to the airfield in about another 45 days. Oh, and they cost about $570,000 in today’s dollars. Do you think that we could possibly do that now? https://en.wikipedia.org/wiki/North_American_P-51_Mustang