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In an inefficient market, the mining rental cost becomes the same as mining revenue, so this scheme almost doubles the attacker's money.
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I think this is and interestin idea. However, while I find it easy to agree with the quoted assumption when we are speaking about small amounts of hash power, once it's about half the network it becomes dubious. No matter how efficient the market is, if you want to buy half of all the supply you'd be moving the price a lot.
That's why my original argument involves a mojor external event: block reward halving.