What is often considered the largest flaw of the bitcoin protocol is the possibility of the 51% Attack. If a single entity (miner or mining pool) would get the majority of the networks hashrate, they would have a the capability to control the network and manipulate the public ledger to their will.
It is worth to take into consideration that getting the majority of the hashrate would cost an enormous amount of money. More than what could be gained from controlling the network. In the event of such an attack the trust and confidence in the currency would dramatically decrease and by pure logic the price would drop. This decrease the incentive to actually strive for majority control of the network.
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