y take their place, and more investment would be needed to keep the attack going.
The best way to make Bitcoin useless is to mine empty blocks and make it impossible for anybody to make transfers, which would make Bitcoins worthless.
It would cost about US$350-400 million upfront to produce enough Bitcoin miners, and about 16 million per month to make Bitcoin useless. An attacker might be able to make some of that back by shorting the market, but even for a large bank, this will be difficult to raise.
The community would have a few tricks up their sleeves to render an attack useless, which would require further investment to keep it going.