Defensive Coordinator for Seattle is a gateway to a future Head Coaching job. For that reason, there will be no shortage of talented individuals available. Promoting "in house" reinforces the lure. Take bright, young coaches and mold them, promote them within the organization, then turn them loose upon the NFL.
I compare it to John Calipari at University of Kentucky churning through "one and done" basketball recruits. Pete Carroll knows that creating a path of success for others will perpetuate success for himself. It is a proven business model that transcends sports and is a desired attribute for leaders within the military as well.
I expect that Pete will have a decent "coaching tree" before it is all said and done. I also expect that at the time he eventually retires, he will recommend an "in house" coach to Paul Allen. He may be grooming the next head man now.
A lot of speculation, but I think it eventually plays out that way.