I don't really see how it could be that, since manufacturing in the USA is more valuable today than it has ever been, has a higher output than any other country but one, and more than some other continents, and makes up 1/3rd of GDP.
The GDP was relative, so even if the pie did get larger it doesn't mean the ratio couldn't go down.
"Manufacturing gross output and value added shares of the economy declined steadily between 1997 and 2013, as shown in Figure A. One reason for this decline is the rapid growth of manufactured imports, which have reduced the demand for domestically manufactured goods. Total imports of manufactured goods increased from $744 billion in 1997 to $1.83 trillion in 2014, rising from 8.6 percent of GDP in 1997 to 10.9 percent in 2013. Had it not been for the increase in manufactured imports, and of the U.S. trade deficit in manufactured goods, manufacturing output and GDP would have been significantly higher in 2013"
