I think you are missing the point. Right now every account can vote for 30 witnesses, while only 20 make all the decisions around here. That means that if any one account accumulates enough stake they can vote in every single Top 20 witness, which is pretty much exactly what we have today! The Top 20 witnesses are all basically being decided with the vote of the two largest accounts. That means, in effect, we have the direction of the platform basically being decided by two people, yikes!
Changing the vote limit from 30 down to 10 or even better, down to 5, we would likely greatly decentralize the Top 20 witnesses.