I already said that.
He's not as productive or as fast as Kessel. I disagree about defense, I think he's better in that department. That said, you trade a disgruntled player for a 25 year old who only 3 years ago had a 30 goal season. Who has carried the offense of one of the worst offensive teams in the league. I think if you put him alongside Malkin and Rust, he'll do absolutely fine.
The Penguins also get a good defensive prospect. Granted, he's about 3 years away from being NHL ready, but the Penguins get younger. That's what Rutherford has wanted to do. He managed to swap a disgruntled player for a younger, talented player that isn't a long-term investment. There's no commitment if Galchenyuk doesn't fit, then you don't re-sign him. If he does and and wants to remain with the team, great, bring him back. I think it was a solid trade for both sides. You're not going to win every trade in a landslide.