The big stagnation is commonly said to have begun in the 1970s. There are many theories of possible causes that are already discussed, but haven't heard blaming Nixon before. Especially I haven't heard blaming Nixon going to China.
Timewise, that makes Nixon going to China in 1972 is an event that happened at the right time to explain the change in innovation.
Maybe it's crucial for innovation to have basic economic production to be done by skilled labor which can think of better ways to do it instead of outsourcing it to the other side of the globe?