You seem to be arguing from the claim that there are exponentially diminishing returns within a certain range, to the claim that there are no phase transitions outside that range. You explain away the one phase transition you've noticed (ape to human) in an unconvinving manner, and ignored three other discontinuous phase transitions: agriculture, industrialization, and computerization.

Also, this statement would require extraordinary evidence that is not present:

every even slightly promising path has had people working on it