I suppose that depends on your perspective, but for me there was a major downturn in the middle 1970's. I suppose the earlier introduction of "the Nashville sound" a decade earlier started the trend, but there was a lot of good stuff still turned out and the older stars still had some control of their recording. After that the recording industry and the radio stations combined to try and control what the public should hear, rather than responding to what they wanted to hear. That left stars like Cash and Jones outside the box and they got a lot less airtime. This airtime was then filled with the industry created new stars as the older music was squeezed out.