if these values are really at the forefront of their culture, you have to examine why after 10 000 years their society remains relegated to obscurity.... Or even why they are unique to the piraha. How is any other amazonian tribe is less content, less happy, less harmonious with their surroundings, less apt to suicide, more covetous and more embroiled in anxiety for the future due to taking on ideas of spirituality/religion?You mean their current position as a content and happy people, living in harmony with their surroundings, desiring few material possessions, not worrying about the future, and never killing themselves?
Alternatively, their (apparent) esteemed social uniqueness could just as easily be attributed to the absence of numeracy in their culture.
(actually it appears from the wiki on them that they do have belief in spirits but that they are determined to be physical things ..... I know that western analysis of animism is frequently inaccurate, so its not even clearly established that they are precisely an atheistic culture).
In short, there are numerous problems with your proposal.