Thnx for your thoughts ... Stax - as described in detail by Rob Bowman in his book - started as a small Southern Soul label and both the expansion (artistically and economically) and the myriad of "hits" in the 60`s - based on superb songwriting - simply happened .... while the Memphis productions were textbook raw Southern Soul, the early diversification with Don Davis stepping in was a first change in direction .... at the end of the the decade a more pro-active search - you could say enforcement of - for top selling records started and the story how songwriter Isaac Hayes ended as singer (aka "try and error") is quite telling ..... the industry changed, innocence was lost and and money (greed) increasingly beacame the name of the game .... Stax subsequently concentrated on a handful of top performers and other acts started flying under the radar marketingwise .... so no disrespect to the productions/efforts in the 70`s, but if listening to the "Complete Stax (Volt) Singles" compilation in chronological order gives an audiblle documentation of the changes ....
You quote of "Johnnie Taylor arguably became the blueprint for the further development of Southern Soul" made me think, but obviously there is no definition of "Southern Soul" and to me a genre of "Soul/Blues Crossover" would more apply ....
The collapse of Stax happened before the heydays of Disco, so we will never know how the company if still active would have mastered this challenge ....
Nevertheless the "Stax Story" is one of a kind and the heritage remains treasurous ....