I don't think there is anything urgent, and they will almost certainly re-sign for another 3 or 4 years. However there are other factors to consider.
1. I wouldn't class the Sedins as generational, but they are certainly elite players. In each draft, there are maybe 1 or 2 players that have the potential to have the kind of impact that Henrik and Daniel have.
2. Their production has fallen off of their peak (100 - 110pts). IMO, a successful NHL top line has to have an 80+ point play maker and a 30 goal scorer. The Sedins right now are right on the edge of that. I think their numbers are down a bit this season due to the lack of a pre-season and period of games where, due to injury, the Canucks played a very safe, defensive oriented game. But I would be pleasantly surprised if they have another 100 point season between them.
3. If the Sedins become a 2nd line, is the Ryan Kesler line a first line? That remains to be seen. If Kesler can return to his 2010-11 form, and Kassian can become a 25 - 30 goal power forward, and one of Higgins, Hansen, Raymond can consistently put up 25 goals, possibly. But RK17 finishes almost every season injured (broken jaw, broken finger, hip/labrum, shoulder ...).
4. I like our prospects, but realistically, we don't have a single prospect who is a sure fire top line forward. But that I mean someone who can carry a line and put up 80+ points and/or 30+ goals. Jensen could be a 1st line player, but as a winger, he is not going to carry the line offensively. Gaunce - looks really good as an overage junior, but it's a little early to slap a C on his chest and expect 80 points out of him. Jordan Schroeder looks to be a 2nd/3rd line center, who if he continues to develop on both sides of the puck, could be a future Derek Roy (a 1st/2nd tweener). Kellan Lain - seems to have 3rd line centre stamped on his forehead.
5. Anyone we draft this spring is likely to be 3 years away from having an impact. Because of our depth, we don't need to rush players or put them into situations where they will fail. However, when there is a deep draft, like this one is expected to be, you have to think to the future of our offense (and our defense as well).
6. Duh - there are 2 of them. As they have always played together, their production is pretty much in lock step. It is reasonable to assume that their skills deteriorate at the same pace, so when 1 of them retires, they likely both retire.
So considering all that, we do need to plan for succession - over the next 3 years, we need to have players who can step up and challenge the Sedins for top line minutes and roles.