Sale had ample opportunities to show us what he can do over the last 4 years, and he was just never able to get out there and do it. Had he stayed and gotten injured again, which would have surprised no one, he would have walked away at the end of the season, looking for whatever deal he could get.
He gave the Sox a few great seasons, but those are in the rearview mirror and no one expected him to be that pitcher again. Atlanta was in a better position to give him a chance, because if he did get injured they have more MLB-ready pitchers who can step in for him. He was either going to stay healthy and give them a good season or, at a minimum, lessen the workload for Bryce Elder or AJ Smith-Shawver or Hurston Waldrep or whoever else was stepping up for their AAA squad. Boston simply did not have the depth behind him necessary to take the risk of putting him back on the mound.
This was a very smart deal for both teams...