Sorry, I just don't buy any of this I'm afraid.
"Twist And Shout" was a wonderful freak of a recording - ONLY John could have delivered that incredible version (too much coiled, controlled power there for screamer Paul to handle) and it's doubtful that even Lennon himself could have replicated the recorded version on any other day (he even admitted as much) and certainly no live Beatles performance of this song ever matched the sheer might of the album version. Consequently I think it's unfair to use this as a yardstick by which to measure other vocal performances.
Although he outgrew the "Dizzy Miss Lizzy" type of songs, John retained his peerless vocal prowess until the end of his life in my opinion. He was neither writing nor performing primitive, gutsy belters like this by the 70s; his style and repertoire had matured and broadened and I do think some of the solo work if anything vocally surpassed - in phrasing, tone, range and subtle control if not raw strength - anything he did as a Beatle. Various live versions of "Cold Turkey", "Instant Karma!", "Well Well Well", "Gimme Some Truth", "Power To The People", "New York City", "Meat City", "What You Got", "Rip It Up/Ready Teddy".... I find no dilution of power there.
But his shift towards generally wispier vocals was evident as far back as the later Beatle days. He'd been the battering ram within the group in those early, energetic years and I think he realised he no longer needed that, he became a more introspective, thoughtful, contemplative artist and bellowing R&R became a rare, nostalgic indulgence. Primal scream therapy showed his lungs could still deliver when the occasion demanded it. Paul retained his own considerable vocal talent by honing it on the road - he still needed those screamers with which to wow the audiences (who had long since lost their appeal to John).