His destiny seems to be to get traded and become a bottom-third starting PG.
This.
Reggie Jackson 2.0
below average shooter and starter, good playoff performance vs. SAS (?) and unhappiness with OKC backup role
It's not a good comp. Here's Rozier's 3rd yr vs Jackson's 3rd yr: http://bkref.com/tiny/4B8HL
Rozier is not the finisher Jackson was, but he's as good or better in all other facets of the game.
Just looking at the numbers you posted, Rozier's ppg is lower,his apg is lower, his FG% is lower, his FT% is lower, and their steals and blocks are pretty close to identical. It looks like Rozier is a better 3P shooter and rebounder and that's it.
As always, in a vacuum, you can use these kinds of stats to compare players, but Jackson's situation in OKC was so different than Rozier's in Boston.
Jackson was given the keys to the offense when Westbrook was off the court. Their offense allowed their primary playmakers to run pick-and-rolls/isos to create opportunities. This raised Jackson's stats.
Rozier has never had that opportunity (except in the playoffs last year). He has always played with Irving, or Smart, or Tatum, or Hayward. He plays in a team system.
Admittedly, Rozier is not as good in the pick-and-roll as Jackson is. But he brings an energy to the team than Jackson never had. He has a magnetism. He has the explosive scoring ability to score 8 straight points and tip the game.
He's also a much, much better defender and rebounder than Jackson. The difference defensively cannot be easily quantified, but if you watch Jackson play, you realize that his defense constantly compromises the entire team's defense.