On the right side PK plays over 25 minutes a night and Ellis is close to 25min (same on the left with Josi and Ekholm). That does not leave a lot of minutes for their 3rd pairing. Moving Subban opens up minutes for Fabbro. He will likely be paired with either Josi or Ekholm.
Fabbro's showing at the Worlds, where he started as D7 and kept on increasing his role as the tournament went on, probably comforted them that he could take on a bigger role next season.
As for NJD Taylor Hall at the end of the season was talking about how they have the culture that is conducive to winning - the work ethic, the speed, the team culture but are missing talent. Shero concurred and said his priority this summer will be to add talent. They do that with Subban.