Antonio Dawson is one of the original characters of Chicago PD, so it was totally unexpected when he left the series at the end of season 6. The Chicag0-set police procedural has been on the air for nine years now. While Intelligence has mostly kept its core team, it has also gone through some personnel changes, including the sudden departure of Dawson (Jon Seda) from Hank Voight's (Jason Beghe) team.
Long before Dawson joined Intelligence, he and Voight already had a history together as he was the one who nabbed his future boss in a bribery sting. As he usually does, Voight was able to get out of prison and, through a deal, was even handed his own department. Despite their less than ideal past, Voight's first hire as Intelligence leader was none other than Dawson. Throughout his stint in the Dick Wolf-produced, Chicago PD, Dawson acted as Voight's second-in-command, taking over the team whenever its Sargeant was occupied with other matters.
Voight and Dawson cannot be any more different, that's why they worked well together as Intelligence leaders. While Voight has no issues crossing lines and breaking the law if it means serving justice or for the sake of people important to him, Dawson is a stickler for the rules. So, when Dawson was effectively written out of the show in Chicago PD season 6, it was a huge blow to the series' cast dynamic. There was no clear reason given as to why Jon Seda exited the police procedural, but according to a report from Deadline, the decision stemmed from the feeling that the character had run its course. Instead of keeping him in Chicago PD without any fresh ideas on the table, NBC decided to write him out of the series. Seda hasn't directly commented on the matter, although he did say that he's nothing but grateful to play Dawson.
Assuming that this is really the reason for getting rid of Dawson, it would explain the sudden change in the character. In Chicago PD season 6, the Intelligence detective developed a substance abuse problem following a shoulder injury. He tried to keep it from his co-workers but went to great lengths to sustain his addiction. Despite Voight's efforts to help, he decided that it was best to walk away from the force, enter himself into a rehab facility, and relocate to Puerto Rico. Clearly, Seda’s departure is different from Sophia Bush’s Chicago PD exit. The decision to introduce this drastic change is arguably the reason why Dawson stopped resonating as a character. Had Chicago PD kept his original characteristic, he would have continued to be an interesting member of Intelligence, especially in contrast to Voight.
For years, Dawson was the moral compass in Chicago PD. Since he left, there hasn't been anyone to clearly take that place as everyone seems to be willing to follow Voight's ways considering that for the most part, his intentions have been good. Thankfully, with the way Seda's character was written out, there's always a way to bring him back, either as a guest or a series regular.
Chicago PD will return for season 10 on NBC.