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After mining rewards go down to 0, it is possible miners will not pick up transactions with 0 fees. In that case, could someone create millions of transactions with no fees, to congest the mempool and slow down the network?

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    The answer to your question is no (I'll write an actual answer latee to explain), but just to know what I have to focus on: why do you think mining rewards have anything to do with this? Sep 7 '21 at 22:19
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    This is not a change in the future; already today, and for a long time, miners usually prefer higher-fee txns, unless you or someone incentivizes some miner(s) out-of-band. This is why wallets automatically suggest (or simply use) a fee rate based on recent successful txns. See as Stack already suggested bitcoin.stackexchange.com/questions/69030/… Sep 8 '21 at 6:08
  • @PieterWuille the reason mining reward going down to 0 was relevant to this question...is because without mining reward and transaction fee, the transaction has no incentive left and thus likely won't be picked up by any miner.
    – septerr
    Sep 10 '21 at 21:42
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could someone create millions of transactions with no fees, to congest the mempool and slow down the network?

Most nodes will discard and not relay any transactions with a fee less than their configured minimum relay transaction fee.

As https://en.bitcoin.it/wiki/Miner_fees says

Relaying

The reference implementation's rules for relaying transactions across the peer-to-peer network are very similar to the rules for sending transactions, as a value of 0.00001 BTC is used to determine whether or not a transaction is considered "Free". However, the rule that all outputs must be 0.01 BTC or larger does not apply. To prevent "penny-flooding" denial-of-service attacks on the network, the reference implementation caps the number of free transactions it will relay to other nodes to (by default) 15 thousand bytes per minute.

Settings

Setting Default Value (units)
txconfirmtarget 2 (blocks)
paytxfee 0 (BTC/kB)
mintxfee 0.00001 (BTC/kB)
limitfreerelay 15 (thousand bytes per minute)
minrelaytxfee 0.00001 (BTC/kB)
blockmaxsize 750000 (bytes)
blockminsize 0 (bytes)
blockprioritysize 0 (bytes)

So there already are countermeasures to denial of service attacks .

None of this has anything to do with mining rewards. I don't doubt miners will also ruthlessly prune any transactions that don't serve to maximise their profits.

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