1) Currently difficulty is at ~10,000 TH
2) Single 1 TH mining box cost ~$1000 to manufacture (reality is much less than 1k, but for easy calculation lets use $1k)
3) To destroy the bitcoin network you need to control 51% of the network.
4) To bring 10,000 TH (assume evildoer wants to be thorough) online it will cost: 10,000 * $1000 = $10million, add on another $10millon for R&D, $20million total.
5) Goldman saches makes ~$30million a day in NET revenue (same with most other banks)
6) It will cost a SINGLE bank HALF day's net revenue to wipe bitcoin out - less than their office supply budget.
1. No one can get 10,000 such mining boxes manufactured in 3 months, and it is very likely in 3 months the difficulty will rise another 10 times
2. Even if someone manufactured 10K such mining boxes, they need a HUGE site to setup those boxes, and 10MW power supply at least, many cooling towers...
3. Combine 1 and 2, anyone who tried to do this will most likely end up with commanding 5% of the network hash power after 3 months with a huge cost
4. Someone might want to manufacture 100K such boxes to stay ahead of the curve, but then the scale of the site will rise to another astonishing level and they might need a small nuclear power plant to power all those ASICs and cooling towers, all these could take years to build
5. Finally, even if someone successfully commanded 60% of hash power, what he actually can do is very limited. He won't be able to affect existing hodlers' coin, just make the future transactions less trustworthy, so once people find out, they will stop transactions and hold their coins and wait for the situation to resolve