That's a lot of disinformation; while Willie had a fair amount of success as a songwriter, his solo career never ever really got off the ground during his time in Nashville in the first place; he didn't fit their mold, and was tired of playing their game. His style was too idiosyncratic, too jazzy. He found that the best response he got on the road was in his home state of Texas. So he moved back, just as a portion of the non-traditional long haired youth culture was starting to embrace country, by way of the Byrds/Gram, Poco, Nitty Gritty Dirt Band, New Riders, and so on. And they embraced him, with his rough around the edges vibe, although the 'outlaw' thing was yet to happen.
As far as Johnny is concerned, "FPB" was first released in the mid-50's, on Sun Records; a modest hit then with limited exposure. His career hadn't 'fallen off' in the late 60's; it was going down the tubes way before then with his private drug hell, but for the grace of June Carter, he emerged from it alive and repurposed. As he had always felt an affinity for prisoners, he felt strongly about recording a bare bones live album in a joint, against the wishes of Columbia Records. But that album, released in 1968, resonated in a way no one could have foreseen(except for him); you might see the 'man in black' veneer as schtick, but it represented his new found sobriety, and seriousness of the times. Not to mention possibly a reaction to the 'Nudie Suit' glitzy look of the Grand Ole Opry scene. There was always a bit of darkness running through his music, going back to the beginning of his career, so I don't see it as some sort of calculated PR maneuver. In both cases, both men had more success when they just became who they were in the first place.