If this were actually the problem, it wouldn't be the 'welfare' state type of nations that are catching up to the US. What actually happened is there were this thing called World War II and during this event there were big planes called bombers, that dropped things called bombs on places called factories where stuff is made. Not only were lots and lots of people killed, but the European and Asian economies were ravaged. In America, the factories were untouched and most of the war casualties were soldiers rather than civilians. The labor force expanded and the economy was strong. As a result America was the dominant market force for the 50's,60's and 70's while the rest of the world rebuilt and recovered. Even in the 70's and 80's the rest of the world was catching up with many countries experiencing much faster GDP growth than the US. In the 90's it was much the same. Now, the economy is seeing the consequences of other countries growing their economies faster for a period of 40 years.
Also, during the heyday of the United States, the top tax bracket paid over 67% taxes, now that number is well under 50%. So if you believe that was the heyday of the free market you shouldn't accuse the government of market interference for trying to raise taxes on the rich.