Yes, I know that you do not see anything wrong with that picture. The top 6 companies have 60% of the hashrate. What could possibly go wrong? As long as there is sand around...
Yes, but you would be saying the same thing if it were 10 with 60% or 20 with 60%. These are pools, miners can re-direct their rigs anytime.
There is no magic number. The fewer companies you have in the right half that chart, the higher is the risk that they will collude to act as one entity, each giving up their immediate gain (reward for getting the next block appended to orthodox chain) for a greater long-term gain (such as a larger reward per block). Why would their affiliated miners defect? If the change will mean more money even for miners who are not in the cartel, why would they resist the change?
Not long ago there was just one pool, GHash.io, with more than 50% power. Bitcoiners were scared, but considered the probem solved when it shrank to less than 50%. No one asked where those miners who left the pool went. No one asks who are the owners of all those pools. How many of those big miners are in the Bitcoin Foundation, paying Gavin's salary?
They are not going to conspire to destroy BTC, what would be the point.
In the real world, cartels and monopolies do not form to destroy their markets, but to extract more money from their customers than they could if the markets were free. And there are plenty of things that a majority coalition of miners could do in that direction.
