Any concentration of miners is problematic. For it always opens up new ways for a system to be abused. Imagine ten holes drilled in a beam. If randomly distributed, the beam will hold up weight. But if these same ten are connected, in line, the beam is almost cut through, and will fail under the slightest weight. Without decentralization the value proposition of bitcoin, etc, is indeed not high.

If this lack of actual decentralization is true, bitcoin is insecure. Not good. A payoff matrix that discourages collusion will have to be included in future cryptos. One solution tried in Steam and Eos is that mining is delegated. So even highly concentrated mining efforts can be kicked out, if they play around dishonestly with a block. Assuming voters delegating notice the behavior that is dishonest and actually bother to vote it out. (Perhaps a big if.)