It's a fact of the cap era that you're going to lose players from very good teams. The thing is, you have to know which players you should keep and which players will walk. Then, you need an eye for developing talent to fill those spots on the cheap.
The Bruins for the past few years were built on a lot of character players. I think we can agree that it wasn't built around 'high end' talent like a Toews or Kane. They were built around gritty veterans with experience. Role players. Everyone played their roll and the team was greater than the sum of its parts.
When a team which is 'flat' in regards to talent starts to come up for contract extensions, you're going to have trouble. Your 'top end' talent isn't going to be up to the standard of the rest of the league, but you're still going to have to pay them like they are. That sort of overvaluation tends to push it's way on down the lineup. The cream of your 'mediocre' crop rises to the top and gets paid. Soon, you're paying your role players more money to keep a core intact, when the only reason they were successful was due to the players they had around them.
Your top players weren't the ones driving.
I think a lot of this comes down to drafting and the Seguin trade. We haven't been able to fill spots with young guys effectively enough, so we've had to stick with some overpaid veterans too long. As for Seguin, he should have been kept as a 1-2 punch with Bergeron. If we needed to move a contract out, it should have been Krejci (as much as I like the guy).
That way, you start to build the team around real top end talent and have those two cogs of your offense like Chicago.