^i think that the percentage of DVC owners who trade out their points is very, very low. It is well known that trading out is a very poor use of points, and people who buy DVC buy it so that they can stay on Disney property. I would say it is a real reach to imply that DVC "needs" RCI and not the other way around, particularly because Disney already swapped II for RCI and I'm sure would not hesitate to make a similar change if they felt it beneficial.
Also, DVC is designed for, and for the most part achieves, 98% occupancy year round. There is no "low sales" time during the year.