Here's the problem with that analysis. We already have ample evidence that:
1) Weber's offensive game doesn't benefit from defensive defensemen. Weber (and the entire team's) offense was putrid when he was stuck with Emelin as a partner. And lest anyone use the excuse that Emelin sucks (assuming Alzner is much better), it was very much the same story the year before in the short amount of time he was separated from Josi. We know Weber. He's at his best when he has PMD partner. Josi, Suter, etc. Alzner isn't going to free Weber fundamentally change the way he has always played the game and take more chances than he usually does in the offensive zone. Especially since...
2) Alzner kills offense from his partners. Whether its been with Carlson or Niskanen or Green, Alzner has always dragged down the offense of his partner and their ability to apply pressure in the offensive zone. He'll be 29 at the beginning of the season and its even more ludicrous to expect him to change his game than it is to expect Weber to change his.
Also, Alzner and Weber have good first pass conversion percentages because they make the safe play. And in Alzner's case in particular its because he rarely makes the 1st pass out of the zone. His 1st pass is to his partner. They're good at exiting the zone, but that doesn't mean that the plays they start are going at gaining the offensive zone. Safe plays out lead to less space for forwards by design. Fewer odd-man rushes, less creating havok in the offensive zone and less zone entries. This has been by Bergevin's design and is a big part of his Canadiens. Safe plays at the expense of offense. Its not an approach I agree with, but its clearly the approach Bergevin wants to take.