The math doesn't work that way because honest miners amortize the capital cost of their specialized equipment over long periods of time. An attacker undermines

The math doesn't work that way because honest miners amortize the capital cost of their specialized equipment over long periods of time. An attacker undermines the network's value and so must operate on shorter timescales. The capital costs of an attack could perhaps be partially recovered via resale, but getting order ~50% of the existing network's specialized hardware might prove prohibitively difficult even given sufficient capital. [edit] And renting existing miners' equipment would be difficult because you would have to: figure out who they are, convince them to point their equipment to your pool, run your own dark pool servers to handle data from tons of miners, run infrastructure to make payouts to these miners, hope that exchanges don't have reorg procedures that prevent you from accomplishing your double spend, hope your chain reorg doesn't panic the market for long enough for you to double spend exchange to another coin that won't be tanking (due to the reorg).