It's not all about TDs. Kenny had 13 TDs in the last 25 games. Ok, that's bad. And he had 50 sacks which is pretty good.Yes, giving him 25 starts. Throw more than 13 TDs and he's still the starting QB.
Each sack takes 1.5 points off the board.
Fields had 29 TDs in the last 25 games. But he had 76 sacks. The extra 25 sacks means you deduct 37.5 points, which is over 6 passing TDs. Then you have to do the same with INTs (Fields had 7 more) and fumbles (Fields had 13 more).
So the net math is 10 passing TDs minus 20 more turnovers.
This isn't fantasy football. The Steelers won a lot of games where Kenny wasn't throwing TDs because he didn't give up an extra sack and INT/fumble per game. Those negative plays are absolutely killer to a team's win rate.