Hayes is a bombthrower who carries water for whatever argument suits him, whether out of conviction or just for the sake of argument (or because the argument has been assigned to him, while Murphy or Sielski make a different one). If Kingery's name was "Wentz" it would be a completely different column.
I think the Phillies' main issue has been the lack of consistency across the board, not just for Kingery in particular, though all the other moves invariably affected his position and playing time. Every decision they made involving Crawford, Franco and Segura, as well as their shift from Machado to Harper, also scrambled the picture for Kingery. As did COVID obviously. Not only do we not know what Kingery might have been like last year without being sick and injured, but he actually would have had 40-60 games at one position (either 2B or 3B) before Bohm came up. I would still love to see him thrive as a Davis/Zobrist/Merrifield type player and agree that there is absolutely no reason to think playing one position would fix his hitting, even if it did mean more time to focus on that instead of fielding practice at multiple positions.
And from what Kingery has said it's not at at true that Kapler and Mallee messed with his hitting. His current messed-up swing and power-happy approach is the one he developed with a private coach in 2017 and the one that earned him this contract. But its holes began to show once he started facing better pitching.
Hayes is probably right about one thing - having made the commitment to him like they did, they probably should have just traded Cesar - or even turned him into the supersub. In that sense Kingery was a victim of his own talent, he had the better defensive toolbox to play on the other side of the IF even though Cesar used to be a SS too.