Comparable. Gretzky was replaced within two seasons in Edmonton-1990 SC. Crosby more than adequately replaced Lemieux.Boston never replaced Orr.
What? Okay, there's a lot wrong here.... Where do I start?
-- "Gretzky was replaced within two seasons in Edmonton".
That's a humdinger. First of all, Gretzky was "replaced" (if that's the word) by Jimmy Carson -- who was so intimidated by the Dynasty-players he ran from the team -- who lasted one season. Sather's brilliant trade of him for Klima, Murphy, and Graves gave the Oilers an extra forward line, which, since they had Messier in MVP mode and still had Kurri, Anderson, Simpson (and emergent Ranford), allowed them to get in one more Stanley Cup before they came crashing down hard.
Oilers' win-percentage last three Gretzky seasons: 67.5%
Oilers' win-percentage first-three post-Gretzky seasons: 52.9%
Bruins' win-percentage last three Orr seasons: 66.6%
Bruins' win-percentage first three post-Orr seasons: 69.2%
-- "Crosby more than adequately replaced Lemieux"
First of all, I can't really see this as a "replacement" since Lemieux's last period of prime-play combined with Pens' team success was 2001 at the latest, and most probably 1996. Crosby appears in 2005, which is like a totally different era, not a 'replacement'.
Second, while Crosby is arguably the best player of his era, he is in no way at Lemieux's level of skill or peer-dominance, making it odd to think of him as replacing Mario.
-- "Boston never replaced Orr".
That's true, I suppose, but as the team's hugely impressive record from 1975 to 1984 shows, they didn't really need to replace him to keep winning.
The fact is: The Oilers won a Stanley Cup without Gretzky, the Pens stayed near the top of the League and tops in offense without Lemieux in the 90s, and Boston continued as one of the best teams in the League for many years without Orr (in fact, they improved in 1975-76 when he was out). No one player is bigger than a team, as all those guys well know. But that's not the same thing as replacing the player. You cannot replace a Gretzky, Lemieux, or Orr.