I think the so called "decline" of the Sedins is overblown and can be attributed to two driving factors:
1) They are indeed getting older, and wear and tear on the body will reduces offensive effectiveness and output; and
2) They are no longer the beneficiary of favorable offensive zone starts.
When AV was at the helm, he utilized aggressive zone-matching. Manny and Kes were given the hard defensive starts, and Kes was always matched up against the other team's top line. The Sedins were given the prime offensive zone starts (the same way Sid, Ovy, Stamkos, and other offensive players are), where if you win the face off you can instantly create a scoring chance rather than having to win the faceoff and then advance the puck and gain the zone. Statistically speaking, every offensive zone start produces an average of 0.8 more shots than a defensive zone start, so if you constantly apply that strategy the impact is huge. If you give a guy an extra 10 offensive zone starts a game rather than defensive zone starts, the team will produce an extra 8 shots while that player is on the ice which over the span of a season will boost his offensive production.
But last year Torts came up with the bright idea to start them in the defensive zone a lot more, and force them to spend more energy on defense and transition rather than focus on what they are best at, creating offense inside the offensive zone. Dale Hunter attempted to try the same thing with Ovechkin a few years back, and he had his worst offensive season ever. Zone starts and player usage can honestly have a significant impact on offensive production, we're talking a swing of 20-30 point potential.
With the new coach, I think we'll see the aggressive zone start strategy brought back, and therefore the Sedins will rebound. They have aged so there won't be any more 100 pt seasons, Kes isn't around to handle the other team's top line which will hurt production somewhat, however they should be approaching PPG if they find some decent chemistry with Vrbata, or if Burrows game is rekindled.
2014/15 Projection
Assuming they are given 6 more offensive zone starts per game (2 per period), that will produce 4.8 (6 x 0.8) shots per game while they are on the ice. Assuming a 40% chance a specific Sedin gets a point on a goal if they are on the ice, and the team's shooting % is 10% which is around league average, that will produce 0.192 (4.8 x 40% x 10%) more points per game for each Sedin, which equates to an extra 16 (0.192 x 82) points each over the span of 82 games.
Henrik was on pace for 58 points last season, so if you add 16 points that brings his 2014/15 projection to 74 if he plays 82 games.
Daniel was on pace for 52 points last season, so if you add 16 points that brings his 2014/15 projection to 68 if he plays 82 games.
Further, that was playing with struggling linemates all season, so adding Vrbata or a healthy Burrows should inject a few more points as well, at least 5 - 10 each, bringing the totals to around 75 and 80 for Daniel and Hank respectively.