Getting those ASICs from where they are unused to where someone can use them to attack the network is not a simple task. The only way this becomes possible is if the current owner of the hardware decides to attack the network. Even to buy the hardware will require a large capital input, let's at you can buy $1m worth of hardware and ship it to your location within 7 days (shipping and setup), you then need to run it (electricity cost) and hope that you can recoup costs plus profit by double spending. The only option to double spend and profit is to double spend on an exchange and withdraw fiat currency before the exchange realises, the speed at which fiat transfers work this is highly unlikely.