You truly need both to excel in the end.
The Hawks, kings, and Bruins not only have great defensive centers, but great offense along with many complimentary pieces. They also all have some of the best Defensemen in the league with great-elite goaltending. The Hawks had guys like Kane and Sharp. LA had guys like Williams, Gaborik, and Carter. While Boston had guys like Krejci and Marchand. Defense is hardly the number one piece to a successful team. Even guys like Gretzky had Kurri and Tikkanen. Lemieux had Francis. Yzerman had a whole lot of great defensive support, but all three also brought a huge level of offense.
Crosby and Malkin have 3 stanley cups now, and that teams main core isn't in their defense, it's in their ability to excel offensively while not being liabilities. You simply need that in this era of the game. The reason why Ovechkin doesn't have any playoff success isn't because he isn't great defensively, it's because he has little to no help from any secondary scoring, the teams defense is subpar, while their goaltending tends to get shaky in the playoffs. All three of those teams didn't have any of these problems. Boston rode the wave all the way to the championship that was Tim Thomas. LA had great complimentary players like Williams, with Quick playing amazing. And the Hawks at their peak simply had everything, they were a well oiled machine.