As of now St Louis wins the trade. Here's a more positive take from a Sabres POV.
It's pretty clear there was underlying reasons ROR had to be dealt. We don't know what those are but it's pretty obvious they felt he was working with the group for some reason. If we are better this season, it is possible it wouldn't have happened if ROR was still on the team, hockey is weird like that. So punching a wall might not be the best idea.
Seems they are banking on essentially swapping ROR for Mittlestadt offensively, while shifting his defensive minutes to Berglund and Sobotka and praying it works. Sheary will hopefully be able to take Kane's spot (Kane put up goals but did not work well with Eichel).
The part you're underestimating is going to be the depth of the team... replacing Pouliot, Nolan, Josefsen with real NHLers like Berglund and Sobotka, and being able to move down guys like Wilson or Girgensons out of the top 6 and into a proper role. On the blueline you can take out non-NHL caliber guys like Gorges and Falk and slot in Dahlin and a guy like Hunwick. In net we can't be any worse. From a positive perspective, the top 6 gets faster and will be able to be leveraged more offensively, and the bottom 6 is going to be much better in that it has NHL caliber players.