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What if mining becomes so centralized,than only one big mining company remains and controls all the transactions?They can say "Pay 25% fees ,or we will stop mining and confirming transactions."What happens with bitcoin,if there are no transactions?
At that point Bitcoin would have failed and it would be time to move to another alt coin. Or alternatively, to fork off the main chain with a new proof of work algorithm and go from there.
This is going to change in the future. The bigger hardware manufacturers like AMD and Intel are looking for new markets. If the market for Bitcoin or Crypto currency mining is big enough, they will expand their range of cards that are specifically targeted at mining. We might see these giants going head to head with Chinese ASIC manufacturers.
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I'm doubtful that this will actually happen, but it would be great news for Bitcoin if mining would once again become more accessible for the regular consumer. Big miners will continue to control large parts of the network either way, due to economy of scales and cheaper electricity, but at least hobbyists would have easy access to proper hardware again.