Leon Draisaitl is known as one of the best players in the NHL right now. He has won a Hart trophy and has posted five straight 100 point pace seasons. Despite being arguably the 2nd best player in the NHL, he is the 2nd best player on his team with Connor McDavid dominating.
The pick swap seems nice, Puljujarvi for Sergachev but that is likely what would not have happened. The Oilers would have been looking for a forward to replace Draisaitl as opposed to a defenseman so they are likely ending up with somebody like Tyson Jost, Luke Kunin, or Logan Brown, the three next forwards.
This trade would have sent the Oilers years back and we would likely have seen Connor McDavid unable to will the Oilers to deep playoff runs all by himself. However, the Oilers would have had prime Taylor Hall to play alongside McDavid.