Oh, and I don't think they had a top 15 pick since 2010/11.
Right. The PC/JS Seahawks have never traded away a top-15 pick. They used their one top-ten pick to get Okung, and their other two top-15 picks to get Bruce Irvin and Earl Thomas. One very minor correction: Irvin was picked in 2012.
Even Irvin, the weakest of those three picks, has had a career above the average for players picked in the first round, so even though they probably did "reach," the pick turned out to be reasonably good (it's only "probably" because I don't know how they were evaluating players, and there have been several cases when the Seahawks valued a player differently from other teams and "draft experts," and the Seahawks' evaluation ended up looking good). The other two were very good picks, both in terms of what was known at the time of the picks and in terms of how the player's NFL career went.
That said, your point is excellent - they might find themselves in a position where there are multiple guys they like at pick #9 and there's somebody willing to give them good return for trading down a few (or several) spots and the Seahawks think they can still get one of the two.
There's also the possibility that they get to the situation where there's a small drop from the players available to them at pick #9 and the ones they expect to be available at the end of the first or even beginning of the second round, so they make a bigger trade down.
On the other hand, I've read that this year, there are fewer teams than usual that might be interested in trading up, and there are several teams that might be interested in trading down, so it might be hard to find downward-trade opportunities.
This is the first time in a long time that I'm this intrigued by what might happen in the first round of the NFL draft.