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.
3What is your question?– Pieter WuilleJul 3, 2021 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.– WouterJul 3, 2021 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.– user103136Jul 4, 2021 at 11:16
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.
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)
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. Jul 3, 2021 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. Jul 4, 2021 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? Jul 4, 2021 at 20:23
1you 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. Jul 5, 2021 at 14:06
1You should not multiply the percentage but the fraction. And this would only be 1.9 million USD per block if the total transaction value in this block is 21 million bitcoins. (assuming btc replaces usd) Jul 6, 2021 at 6:56