You're missing my point. At the 4, literally ANY actual NBA power forward (and no, Jalen Smith doesn't count, as he has proved he is not an NBA player, at least not yet) is better than the 6'8", 210 pound, slender, frail Cameron Johnson.
Forcing Cam to play that badly out of position hurts both the team (as we saw in the Lakers and Bucks series in the playoffs, and even in the Clippers series to a certain extent) AND Cam (who doesn't get to play his natural position at the wing). AND it hurts whomever is playing Center, leaving him on an island by himself inside to fight for rebounds and try to protect the paint. AND it puts more strain on CP3 and Booker to carry the team, leaving them gassed and susceptible to injury as the season progresses. This team absolutely HAS to find a way to get some semblance of an inside game (on both ends of the court) out of something other than the Center position if it wants any hope of success against the improved and healthy competition in the West this season. Otherwise, the team will have to shoot at least at a 55% clip from 2 and 45% from 3 to win 50 games.
Cam should be playing 20 minutes or so per game at the 3 primarily and at the 2 secondarily. Unless he can put on 10-20 pounds and KEEP it on (which he couldn't do after the bubble), Cameron Johnson is NOT a power forward. In fact, in terms of physique, Mikal Bridges (at an inch shorter but with longer arms and 5 pounds heavier than Cam) is more of a power forward than Cam is, and Bridges at power forward would be an abject disaster.
And losing guys at the end of the roster who aren't going to play anyway in favor of filling an enormous hole for 20 minutes a game isn't going to kill team chemistry, especially given the negative effects on the roster of not doing so noted above. Adding to a rotation to fill its holes is a necessity. Otherwise, why bother even picking up McGee and Shamet? By your logic, we should have kept Carter, Moore and Galloway, forfeited our draft pick, re-signed Torrey Craig and left the void at back-up center as well as power forward.
Power forward was the biggest weakness on the roster heading into the offseason, and it only got weaker with the departure of Torrey Craig. I would gladly sacrifice the 14th or 15th spot on the roster for a key addition to the rotation, in order to avoid being exploited by bigger opposing line-ups the way we were in the playoffs, especially in the Finals.