You're not thinking about any of this at the time the move was made, you're justifying it with hindsight.
Yes, Trenin-Rossi ended up being better than Johansson-Gaudreau-Zuccarello. However, we don't know if swapping Gaudreau and Rossi would have changed things because that line never got touched for some reason.
And saying "moving Rossi away from Trenin when they were working well would have been a bad decision" is exactly the double standard everyone is talking about, because in game 1 Hartman and Trenin worked well, and Hartman still got moved up. But why as it not a bad decision to separate them when they were working well?
It wasn't a calculated effort to get more offense, it was stashing Rossi on the fourth line to keep him off the ice more. Again, the fact that they still didn't get more minutes after they started producing is a testament to that very fact.
And using hindsight to say "well look it worked and they produced" is just a tool to deflect from the fact that the decision didn't make sense at the time, and to deflect from the double standard that Hartman playing well with that line earned a promotion, and Rossi playing well with that line earned a "but they worked well together so why split it up".
Finally, I can't stress enough that Gaudreau was terrible. Perhaps if they had demoted him for playing like shit like they had with Rossi, or promoted Rossi for playing well with the fourth line like they did with Hartman, that line wouldn't have been a complete no show. And I fully expect you to ignore this entirely valid point and say I'm just pushing a "Rossi is a victim" narrative. But you can't really ignore the reality here.