Oracle,
Olynyk can't rip down rebounds. Avery can't hit the roll guy on the pick and roll. Turner needs spacing on the floor to allow him to go one on one. Players are seldom perfect unless they are Lebron or Curry. However, there is a big difference between surviving a Marcus Smart offensive rotation with limited help then the other factors I listed with our other players.
IT's defense is also no where as bad as Smart's offense. It's at least serviceable.
With Smart on the floor the C's this year score 103.0 points per 100 possessions.
With Smart off the floor the C's this year score 104.3 points per 100 possessions.
Statistically your argument that he is killing the offense doesn't hold water.
Avery Bradley had some of the worst shooting charts imaginable for years and has only in recent years started learning what a good shot looks like. He is not a very good off ball nor help defender. He still makes critical errors at times, but he has painfully slowly gotten better over the years.
Olynyk does not rebound horribly in comparison to other players that are used in a similar way. Look at Ibaka, Bosh, Anderson, Fry, Nowitzki, Ilyasova and the like they are all between 7 and 8 rebounds per 36, K.O. is at 7.4. All similar players have seen a reduction in rebounding numbers over the last couple years as their usage has changed on both ends of the floor. You can't rebound if you aren't near the hoop. With that said he certainly could be better, but not like people think, the quality opportunities just aren't there to be a whole lot better.
No matter how much you spread the floor Turner is still not efficient. Unless he learns to shoot 3's at a decent rate he will remain in the bottom 1/3 of all guards in efficiency. Love the guy though and if he can get there he would be excellent to keep as a multi-tool off the bench.
It is insanely difficult to try and properly evaluate players and people(including me) try to do it with miniscule bits of information and the ever bias eye test.