Crypto Briefing said that according to preliminary estimates, at the end of September this year, when the bitcoin calculation reached a high of 90 EH/s, the cost of a 51% attack on it was about $4.2 billion. Surprisingly, this is not much higher than the 42 EH/s in February 2019, when the 51% attack required approximately $2.6 billion. This means that since then, the cost of attack has only increased by 58%, while the computing power has increased by 104%. Therefore, when assessing the security of Bitcoin, the indicator of rising power may be misleading. If the price of the BTC remains the same, then 51% of the attack costs may fall to about $2.1 billion after halving. Ultimately, as bitcoin block rewards are halved again, network security will increasingly depend on cost. At some point in 2140, block rewards will stop completely, which means the network will need to rely entirely on transaction fees. The current daily income from the cost is about $350,000, which is only 2.2% of the daily block revenue ($15.8 million). With the halving approach, the cost will need to increase by 22 times to make up for the lack of income. Otherwise, the computing power will decline and the security of the network will also decline.