In fact, as long as these 20 entities are not attackers, and they have never been
As of today. But that "20" will continue to shrink until someone can control 51%. It's inevitable assuming "all things" stay the same.
History seems to indicate the opposite. I remember when the pie chart was mainly 3-4 pools, and there were a few times a pool DID achieve 50%+. There are more pools today than ever, and it will keep growing.