Is it really so unique? I suppose that both Quechua and Nahuatl (maybe even Mapuche and Aymara to some extent) are in a broadly similar situation in Bolivia, Peru and Mexico. Or is Guarani even more rooted?
By the way, many of the Jesuit Reducciones among the Guaranis were actually within the present-day borders of Brazil and Argentina. I'm aware that a significant part of that area was in Paraguay before the war of the Triple Alliance, but at least some major sections were Brazilian or Argentinian even before that. However, I think that only Paraguay had a lasting imprinting of that historical experience. (The area involved has experienced a relatively large settlement of Syrian and Lebanese migrants afterwards; I find it interesting).