Exact opposite. The US has not improved much since the 1980s. Our times across the board are very similar to our times then with the exception of a guy like Rupp, who ran amazing at this Championships, BTW.
The biggest thing that changed since we stopped improving from the 60s to the 70s to the 80s was some evil brainwashing occurred and we became afraid to race. Our best athletes raced all the time 30-40 years ago. They raced more INDOORS than they race all year now.
Look at two of the most successful stories of this championships: Degrasse, an NCAA runner and Murphy, who exceeded expectations just making the semis.
Now take Jager on the other side of that coin.
The world was stuck in a rut for decades and the US managed to appear dominant. Now the rest of the world is starting to improve and we're still stuck in the non racing, non improving mindset. Put it all together and Cuba has more medals than we do.