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51% of Bitcoin = 10.71 million BTC. If only 10.71 million Bitcoins are in the hands of any individual or group, they may be able to interfere with the entire network in a dangerous way.
Exactly how?
Yeah, so, you are right about decentralization being important, cant argue there. But the idea that a 51% attack means someone literally "owns" most of the Bitcoin? Nah, thats not it. What actually happens is a group ends up controlling more than half of the network computing muscle (the hashrate, if you wanna get nerdy about it). Owning 10.71 million BTC or whatever is totally unrelated to pulling off a 51% attack. The danger comes from controlling the network, not hoarding coins. Thats a big difference.