There's another part of this that's somewhat invisible. Full disclosure - I haven't looked into this in a while. But I believe that some non-Disney timeshares (such as marriott's) still can use Interval International to transfer non-
DVC points to get a DVC room. So there's a percentage of points that being swapped out (for cruises, etc.), but also some points that are being swapped in. So the 20% with this may be a net 15% or even 10%. I really have no idea how much comes into the system this way.