Cooper wasn't with the Cowboys for the first 2 1/2 years Dak was QB. Are you really saying Dak was "absolute garbage" for 2 1/2 years?
As for being "absolute garbage" when Tyron Smith was out, you should check your facts. I assume you are going by the 8 sack game when Green played, which obviously was not Dak's fault, but the overall stats do not support your claim. He sucked against the Raiders and the Giants during that stretch Zeke was out and couldn’t put up more than single digits against the winning teams he played. The team is 7-4 in those games, and Dak threw 17 TD's to 6 INT's, added 4 more rushing TDs and averaged 269 yards passing per game. 2 of the 4 losses were when both Tyron and Zeke were out, and one of them was the 8 sack game that Green was responsible for. For obvious reasons I'm not including the last game of 2016 where Dak only played the first possession or two.
As for Zeke, Dak definitely had some poor showings when Zeke was out, and a couple of others that were decent. I think now it would be less of a problem because the offense was structured then to rely as much on the run as the pass, and last year they opened up the passing game. I think because of that Dak would be better equipped now.