Sure, different players respond differently and some will improve, while others will regress regardless of input from coaching. In my opinion (and for the record, while I have been vocal about this, I recognize that I am no expert... but are any of us, really?), the issue under Cora is that the problems have been systemic. It's up and down the roster, and it's over and over and over. The team is simply non-cohesive on the defensive side of the ball.
Watch a mid-level team like the Diamondbacks play defense for a couple games, and compare them to the Sox in recent years, and the difference is noticeable. It isn't just the plays they mess up, or don't make, either, it's even on the plays they do make that you can see the difference.
Watch a stellar defensive team, like the Rangers or the Cubs, and it's like night and day. And it hasn't been a 2023 roster thing, the 2022 team wasn't great defensively, either, and the 2021 team was probably middle of the pack at best (and a lot of people ignore just how badly that team faded down the stretch). I don't think the blame is entirely on Cora, the rest of the coaching staff is as much to blame for the deficiencies, but when he comes out and says he is happy with it and everyone will be back next year, I think that's a pretty big strike against him, and the organization as a whole.