This is a fair discussion, and you are right that there should be improvement in the offense over the first part of last year because of more than just Cooper.
I do, however, think the scoring numbers you mention are misleading, because there aren’t that many games in an NFL season, and obviously are fewer when breaking down to parts of seasons, so the numbers can be skewed by individual games, and may not take consistency into account. For example, the numbers before Cooper came are skewed by one game the Cowboys won 40-7, but that game was not representative of that part of the season. Similarly, the numbers during the successful second part of the season - after Cooper came - are skewed by a 0-23 loss, which was not representative of that part of the season.
I’m interested more in consistency. For example, the Cowboys scored 27+ 6 times last year - once before Cooper, and 5 times after Cooper arrived. Dak had over 200 yards passing 11 times - 3 times before Cooper, and 8 times after Cooper arrived. Dak’s QB ratings were also consistently better after Cooper arrived.
Granted, Cooper can’t take all the credit, but I think it made a big difference in a lot of ways, including team confidence, and Dak’s confidence, to have that receiver that everyone believed in. He wasnt an inexperienced rookie like Gallup, or a well past his prime player like Hurns, or a career underachiever like Austin, or a one dimensional player like Beasley. He gave the receiving group its Zeke Elliott. That boosts everyone’s confidence.
Dak hasn’t had that Drew Pearson/Michael Irvin/Jason Witten player to pair with, have supreme confidence in and build a chemistry with the way past Cowboy QBs have. Cooper is the first, best chance for that.