The real problem is the inevitable centralization of mining. Year after year, the number of active mining pools go up.
Today's hashrate distribution: F2Pool 20% + AntPool 12% + GHash.IO 10% = 42% of the network. Unknown miners: 30%.
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.
Very concerned about the long-term viability of this experiment.
Very concerned.