Great line of thinking there. Didn't think of the SJ angle.
For me, Pettersson is this years' buy-low top line C that other teams seem to get so often. Look at Dallas' depth, Florida's and how those guys were acquired. Seguin, Duchene, Granlund, Tkachuk, Bennett, Marchand, Reinhart all traded for when their stock was low.
If Boston can turn the Zacha/Mittelstadt money + into Pettersson and have him locked in with Geekie and Pastrnak for the next 8 years while also pushing Lindholm down to the 2nd line, you suddenly have depth in the middle with an option of adding a 3rd line C via UFA to really round out this roster.
Sure, it'll cost but for the first time, the Bruins may have pieces that can make that happen.