Shorting it is the obvious way. You borrow a bunch of that currency, sell it, then use the money to buy hashing power, and steal it back to cover your shorts.
You have to short it by billions and billions of dollars.
Bitmain, alone, made 3-4 billion dollars in PROFIT last year. And they don't control even close to 51%.
Bitmain would have to make enough money off of a single attack so as to cover all it's FUTURE profits.
And then it would also have to not be noticed.... Do you really think all these exchanges would just suffer under an attack that loses them many billions of dollars, without retaliating? No, they wouldn't. And then people would end up in jail.
