From the point of view of native or indigenous peoples in the United States, and for that matter, the western hemisphere, 1491 was the final year in which their cultures and civilizations were allowed to progress in an undisturbed way, free of widespread European contact, conquest and genocide. While there was undoubtedly some random contact with Europeans before that year, especially via Greenland, these contacts were insignificant in terms of cultural impact.
In 1492, Columbus's arrival would disrupt and shatter native cultures, as historians such as Howard Zinn have documented. This process would continue for centuries.
In 1491, native cultures in the United States would have been unaware of Christianity, guns, smallpox, European concepts of private ownership of land and racial hierarchy, and unaware that a change was coming that would completely upend their cultures. While the West routinely celebrates the arrival of Columbus as opening the New World to the Europeans, native cultures look at it as the year that brought disaster to their continent, and 1491 as the last year of prosperity for them. The land that comprises the US would be left largely uncolonized for more than a century, but after 1491, the end for them of life as they knew it was in sight.