Trading for Turner means Al Horford does not return
With Turner onboard, getting Horford back would not have been a priority, knowing the Celtics' big man depth at the time. Besides Turner, the Celtics had the up-and-coming Robert Williams III on deck and Daniel Theis to fill out their big man depth.
Whether or not Turner would have filled the role Horford has since coming back to Boston is debatable, given Turner's reputation as a shot-blocking floor spacer. However, Horford played a huge role in why the Celtics got back to their winning ways in his first season back. Watching him still maintain his ability to play at the high level he has despite his age has been one of the most fun surprises the Celtics have witnessed in recent years.
He's played so well that he has a solid case for getting his number retired in the Boston rafters, especially if the Celtics win Banner 18. That would not have been the case if the Celtics had Turner instead.