I don't think "addition by subtraction" is the right phrase. That suggests we'd get better by doing nothing but removing those guys from the roster, and that's not true. However, I do think we can replace their contributions, and have otherwise upgraded the team.
To counter your point, I agree with what ShaqAttack wrote above. I feel that Turner and Sullinger are the kind of players that put a team on a treadmill.
Maybe this is about Danny getting lucky or rewarded for taking chances. Perhaps the main reason Turner didn't return is because he was offered too much money by Portland. Looking at the box score from their game one, it looks like they got ripped off.
Maybe you are correct my use of the phrase was misguided. The addition by subtraction, to me, is that more dynamic players have replaced Sully and Turner minutes. You are definitely correct about Sully. We added Horford which makes addition through subtraction with Sully impossible to gauge.
So Turner is apparently easier to debate as addition by subtraction than Sullinger?
I am curious if and how Danny knew Rozier, Smart and Bradley would improve ball handling to the point that we could get along without Evan. Perhaps there is a fine line between luck and skill in regards to general manager.
I think letting Sullinger go in theory was addition by subtraction. We are turning into a fast team. Sully was slowing us down? We got faster simply by getting rid of the extra weight?
If we didn't sign Horford, Zizac might have been signed? I concede what ifs are a whole other ballgame. Many believe one of Danny's blind spots has been the center position. I think Sullinger as center was symbolic of that.
Sully has talent, no doubt. It looks like he could have become a better version of the projected Yabusele as skilled, not slow for bulk. Maybe Yabusele is the wrong example. Sully has skills Bass and Humphries are missing, but the latter guys were quick. Sullinger seems to be throwing away his future as an NBA player.
Turner was +10 had 5 rebounds, 5 assists, 1 steal, 1 block, 1 foul, and 0 turnovers. Now yeah, he shot a horrendous 1 of 7, but his team was 10 points ahead with him on the floor because he fills up the stat line. He got Crabbe a lot of open looks with his ball handling and drive and dish skills. That is exactly what he did for Boston and he won't always be horrendous shooting the ball.
There are stats out there that Boston was basically the worst offense in the league last year when Thomas or Turner wasn't on the floor. I don't think you can just automatically assume that Brown, Rozier, and Smart are going to replace what Turner provided.