David Harris has now completed his four year, $36 million contract. At the time he signed it, he was one of the foundational pieces of the Jets' defense. He was a second team All Pro the year before he signed it.
Harris' play declined almost immediately at the start of his contract. While his tackle totals remained high, he was no longer making the same kind of impact getting off blocks and shutting down opposing run games. Harris was never a strong coverage linebacker, but he is now on the wrong side of 30. As his athleticism has decreased, he has become a two down player.
This isn't to say Harris has no role on the Jets. He still is an above average run defender, and there is a spot for a two down linebacker on this roster. Rex Ryan had him leading the team in defensive snaps the last three years, but what that head coach gone perhaps the new head coach would find a role more suitable for Harris.
I don't think Harris is a must-keep player. I just don't see a ton of available players who are head and shoulders above him. The options for quality three down linebackers is limited. I see some players who are roughly equal. Some of these players are younger, which is good. Younger players generally last longer. There isn't any guarantee the Jets can land them, though. I think if Harris is willing to stay with the Jets, take a substantial paycut ($3-4 million max), and sign before the free agency period begins, it might make sense to bring him back. Alternatively, if Harris wants to maximize his earning and pit Rex Ryan's Bills against the Jets, I would be inclined to let him go.