Not sure I understand. There are already fees to get your transaction into a block. That's how the system works. It's not percentage though. Also that would be considered an attack on the network.
You can imagine a miner saying "Oh, you want to move a billion dollars? That's going to cost you more than $3." If there aren't many miners, collusion to enforce price floors does not seem infeasible. It occurs in other industries, like oil and pharmaceuticals, for instance.
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Just like you could imagine whoever needs to move 1 billion dollars would have a decent hashrate so as to not being taken hostage.
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It's an imaginable thing; but having to own your own hashrate to transmit money would be a wholly new paradigm, and is nearly isomorphic to simply paying the ransom.
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Agreed, this is a new paradigm. My prediction is that nation states, corporations, central banks, high net worth individuals etc will all participate in mining, in order to avoid censorship or attacks on consensus rules. Increasing decentralisation further as a result. Like clockwork.
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