Agree in avoiding centralisation. Any idea if there needs to be such a cut off between 20, or however many in the top level, and the rest?
Some kind of smoother descending scale of payments, all the way down, could work. It would scale naturally as the Steem price increased.
Any specific cutoff will run into the same problem. What we need is a sliding scale that adjusts to total amount of active users, so that a large enough Witness group is always available to resist coercion.