Bossy retired early but Was fairly durable the time he was there. He also has a better goals per game.
The reason Bossy has higher career goals per game is because Lemieux played longer. Lemieux was ahead (usually well ahead) of Bossy in career goals per game until Lemieux's final 11 career games (during which time he scored only once) when he was 40.
At 30 years old Bossy had 573 goals in 752 games (0.762 goals per game). He didn't play a game past 30.
At 30 years old Mario had 563 goals in 669 games (0.842 goals per game).
From 31 to 35 Mario only played 119 games, but scored 85 goals (0.714)
From 36 to 40 he only scored 42 goals in 127 games (0.331)