Do you think that a private pool like Foundry will collude with other mining pools for a 51% attack? And which centralized exchange will they attack?
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If I'm honest I don' think so. However having such a large % of the network controlled by such a small number of pools fundamentally goes against the principles of a decentralized network.
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If you do not make transactions during the attack, then your bitcoins are safe.
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Safe from the attack, but (unless you act quickly) not safe from the inevitable market crash that would follow news of Bitcoin undergoing a 51% attack.