I've been through it almost twice now and I didn't notice any mention of two of my favorite singers- Don Williams and Vern Gosdin but as has been noted there were bound to be some oversights. I think it's an important documentary if only for the interviews with some key individuals who are no longer living but it's a whole lot more than that.
Ending with the death of Johnny Cash was a good way to bookend things given his connection to the Carter Family and as Darrel Scott pointed out in "Country Music I'm Talking to You" it's become about "rum drinks by the pool".
And yeah, way too much Hank Jr.