Backes, Rask, Bergeron, Marchand, Krecji are all 30-34 years old. Like you mentioned, Chara is 41 years old now...we saw Carlo take a significant step back last season, and they traded McQuaid for futures to get him in to the lineup this year.
They also lost Rick Nash, Tim Schaller and Riley Nash to free agency.
We're looking at sophomore seasons from McAvoy, Debrusk, Heinen, etc. Could they slump or stagnate like many do in their 2nd season? Potentially.
Let's not forget they had very little hype going in to last season and surprised almost everyone with an unpredictably hot second half. The season before that they were unspectacular, and the two years before that, they missed the playoffs consecutively with this core.