Semin had 2 years at 90-100 point levels. Besides that he was producing at numbers very similar to Erat, with similar linemates. Obviously Semin>Erat, but I don't think the difference is that big (we'll know more as we see Erat more). Also it's hard to argue a near PPG winger>>>>a near PPG center, considering one has Eric Staal as a linemate and one has Troy Brouwer.
Again, Semin is a better player than both Erat and Ribeiro, but I think you're over exaggerating just how much better he is.
You're looking at point production, in all situations (when ES is all that matters when talking about the 2nd line). You're ignoring how much they give the other team during that time.
Semin was much, much better than Ribeiro in that respect. They might put up similar points. Ribeiro will get scored on way more. The line now isn't as good as it used to be.
I don't care about position, and I don't really care about who they're playing with. As the best player on his line Semin's second line was comfortably in the black in terms of chance differential; they dominated at ES. As the best player on his line Ribeiro's second line is basically neutral.
Granted, sussing out how much of that is the players and how much is the coaches isn't easy, but Ribeiro's track record suggests this is what he would be for any coach. And I have to hope Oates isn't that much worse than prime BB.