It's time. In the regular season Bergeron can still go, very much so. After all, he's odds on to win another Selke. But the regular season is just light entertainment. Playoffs are where it gets real, and where a player's ultimate ability to contribute needs to be judged. And Bergy has been declining in that regard. Staal had the better of him last year. This season he was injured, but even so the difference between what an older Bergeron could bring compared to a younger, more physically able and impactful Bennett was noticeable. And that gets to the other point - Bergy's about to turn 38, has a huge amount of mileage on the meter, and the body is wearing out. I think expecting him to be a genuinely effective top 6 center in the playoffs is now asking too much.
I think if Patrice wanted to play next year as a 3C he could do it, but would he want to, and is it really in the long term interests of the team? Doubtful. He's absolutely earned the right to choose when to hang up the skates, and I would never say that he 'must' go. But I just think this is a good time for him to walk away, and to do it while he still has something left in the tank.
My gut feeling though is he'll find it difficult to quit, knowing he's still got something to offer and given how this year ended. I suspect he'll sign for one more year and if he does I'll support him in that.