Very concerned about the long-term viability of this experiment.
Don't be concerned. Even if the "unknown" increases and reached 100%, it is extremely healthy, and was the
normal state of affairs before mining pools were invented! Unknown miners are unknown to each other and can't collude. Yes, you can have a big secretive miner, like 18xf3MQ, but several exist today anyway.
It only would take for a fraction of those "unknown" miners to collude with the three leading pools (assuming they are indeed separate entities) in order to attack the blockchain.
Who is Discus Fish going to call in the "unknown" group to collude with? Are they going to start ringing every single number in the Chinese Phone Directory? You can't criticize the concentration of mining power into a few pools while also lumping in the unknown miners. That is illogical.
The only thing better than unknown miners is the P2Pool. P2Pool has scaling problems, and maybe there needs to be many of them. This is an important development to further improve the mining situation.