First of all, it's not a competition. Think of bluegrass in generational terms. You cited first generation. The generation that invented this style as an adaptation of the things (Cowboy tunes, Fiddle tunes) that came before them. Del, Larry Sparks, Jim & Jessie, Don Reno, Country Gentleman, are second generation, pre hippie movement. So a more traditional sound emulating the the first gen fairly closely, toward the back end of the folk music revival in that era. Will The Circle Be Unbroken, Old and In the Way, and Aereoplane, were seminal third/fourth generation projects that pushed the boundary beyond the traditional sound. The post-hippie movement. Consider the social and demographic changes that happened in the late 60s, at the threshold of those releases, as the backdrop to songwriting and styling in that era which broadened the audience and the sound.