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I think the assumption of a 51% attack is not worth the effort is wrong. Eventually the price of Bitcoin will stabilize. Let’s say at 1 million USD then a diminishing block reward and the transaction fee need to compete with a 51% pool that can make arbitrary amount of profits. The transaction fees will have to pay for 51% of the network to keep everything secure.

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    What is your question? Jul 3 at 16:26
  • How would any reasonable transaction fee keep stuff secure in the end as just robbing people from their money has a better return on you investment. The 51% pool could still pretend to be running transaction just double spend once in a while a very valuable bitcoin transaction.
    – Wouter
    Jul 3 at 20:13
  • The first sentence considers one assumption wrong and then everything else mentioned in the question are assumptions. I think the assumptions and predictions related to bitcoin fees are wrong.
    – Prayank
    Jul 4 at 11:16
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Fee Ratio Multiple = Miner Revenue [Block Reward + Transaction Fees] / Transaction Fees

A low FRM suggests that miner revenue can be maintained without having to rely on an inflationary subsidy. In the below charts you will see lower highs for FRM in every cycle.

FRM-1

FRM-2

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Apart from other reasons why this won't happen. Let's reason if it's worth the effort to do a 51% attack.

So let's say are able to acquire the hardware to form this 51% attack

This would be quite an investment at the current hashrate, which is +/- 100M TH/s

A modern miner like Antminer S19 Pro costs $10k and runs at 110.0 TH/s

So you need 900.000 of these to get that amount of hashing power, which would cost you 900k * $10k = $9 billion. Of course you 'just need' 51% of the hash power, so let's say it's $4.5 billion.

You now can only turn back transactions if I'm correct. So you can pay for something in BTC and quickly fork the network to undo a transaction.

Once you did this, everybody will see this. Probably people will immediately stop accepting BTC now. And you cannot trick other people and have free payments.

Ergo: It's hard to imagine you win back your investment in mining equipment before people realize you cannot trust a BTC payment.

And I even left electricity out of the equation (small costs compared to mining investment)

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  • You have to compare it with the transaction fee's. The transaction fees will have to pay for all this. Would you pay 4.5 billion to secure 7 transactions per second? Now if you double spend a few very valuable transactions there will always be an incentive to join such a pool. Why wouldn't you it has a better return on your investment.
    – Wouter
    Jul 3 at 20:07
  • Ok, so the theory is that by the time the block reward is approaching zero tx fee will be enough incentive. Assume that BTC is a major currency by that time and the fee is still +/- 0.00024 BTC per tx, with 2000 txs in a block, you would still have 0.5 BTC reward as a miner for finding a block. How much is 0,5 BTC worth then if there are only 21 million BTC around (less in practice). 0.5 BTC = 0.000238% of the total supply. An estimate by CIA is there are currently 80 trillion USD. 0,00238% of that alone is $19 billion. So 0,5 BTC doesn't sound too bad.
    – Roy Wasse
    Jul 4 at 20:21
  • Also, you wouldn't join a pool for a 51% attack. You would need to control all the hashpower. Because: how would you convince others in the pool to coordinate together on an attack?
    – Roy Wasse
    Jul 4 at 20:23
  • One more remark: bottom line of my initial argument is still standing. If some entity has the power to do even very selective double-spend transactions, trust in BTC would still be broken. It would be really hard to cover up a double-spend, so the selective part doesn't make a difference I think. And, the entity who has invested most in mining equipment has more to lose than the other miners. Game theory predicts the entity with 51% of hashpower would want trust in BTC to remain, so she can keep mining in the future for a profit.
    – Roy Wasse
    Jul 4 at 20:32
  • you missed a few zero's: 0.5/21 million = 0.0000000238 = 0.00000238 % * 80 trillion = 1.9 million. and that would require you to move all 21 million bitcoins each block.
    – Wouter
    Jul 5 at 14:06

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