In the context of Church history and Book of Mormon geography, the "crumbling of one fundamental assumption" has also had reverberations throughout the whole underpinning of long-held beliefs.
The fundamental assumption was that Joseph Smith linked the Book of Mormon to Central America. That assumption, based on mistakes in history, is crumbling in the face of detailed historical analysis. The entire Mesoamerican theory is crumbling along with it, as are the various rationalizations made by LDS scholars who promoted the Mesoamerican theory for so long.
Terry Givens has been one of the greatest proponents of the Mesoamerican theory. He wrote the Foreword to Mormon's Codex, calling it "the high-water mark of scholarship on the Book of Mormon."
In By the Hand of Mormon, Givens described the Stephens and Catherwood books about Central America. Then he wrote: "Joseph was quick to see how the Book of Mormon had arrived on the scene of this mystery with impeccable timing. Responding immediately to the Stephens account, Joseph wrote back to Bernhisel, thanking him for the "kind present" and ecstatically declaring that it "corresponds with & supports the testimony of the Book of Mormon" ... Picking up the thread of Stephens' remarks, he wrote (or sanctions) these remarks in a subsequent article." Givens then cites the anonymous Times and Seasons articles before claiming "Joseph's enthusiasm for the service that antiquities could render the cause of Book of Mormon historicity" in the context of Central America was the case for the establishment of a museum at Nauvoo.
In reality, the only enthusiasm Joseph expressed was about Cumorah in New York. He didn't write the Bernhisel letter. He didn't write or endorse the anonymous Times and Seasons articles. He never once made or approved of a link between the Book of Mormon and Central America.
Givens is far from the only LDS scholar who embraced the false assumptions about Mesoamerica, but he is one of the most prominent. He and like-minded LDS scholars have given us a series of cascading assumptions that have led to the conclusion that Joseph and Oliver didn't know what they were talking about when the said Cumorah was in New York. This has led to all kinds of mischief, as I've discussed throughout this and other blogs, as well as books and articles.
Crumbling is one of many possible metaphors for what is happening. As the erroneous assumption crumbles, the entire foundation of the Mesoamerican theory will collapse. Along with it, the idea that Joseph and Oliver were ignorant speculators who misled the Church will dissipate. The idea that Brigham Young, Wilford Woodruff, and Heber C. Kimball misled the Church about the repository in the Hill Cumorah in New York will also dissipate. The various rationalizations given by the scholars for events in Church history that contradicted their Mesomania will be exposed for what they were. We'll understand Church history better, and we'll have a greater appreciation for the Book of Mormon as an actual history of actual people that took place in an actual, real-world setting that makes sense and is consistent with what Joseph and Oliver said all along.
We can be happy that this one fundamental assumption is finally crumbling.