so how would they get a large stake? they could place a 100k BTC buy order on stamp. that would be terrible!
more plausibly, maybe they could steal 700k coins from gox. but why would you risk your stash failing to destroy a currency that you would most benefit from if it were successful? Incentive is written in the protocol, and it works outside the protocol too. decentralised cryptos currencies are here to stay, the big players know this. they also know that if they somehow manage to destroy bitcoin (HOW??) a new one will take over and their efforts will be in vein. the more bitcoins that governments and banks own, the more promising the future for bitcoin will be. sad but true.
The banking sector has created a derivatives market some 10-15 times greater than world GDP. The debt pyramid time-bomb that will take down the USD petrodollar system if the US don't ensure that everyone else crashes beneath them. Which I believe they will succeed in doing.
Anyhow, in short, the large banks could just pull the money out a fkn hat if they wanted to. What is to stop a large investment bank from creating a some 'investment company entity' and then lending it (creating out of nothing) vast sums of money to do with whatever it wanted? Answer: Nothing, they do this all the time and this practise was rife in the housing bubble, or rather the mortage backed securities bubble.
Who is to say that they haven't already been doing it? acquiring large numbers of BTC in off-exchange transactions, buying a bit on exchange in order to create bubble conditions, using sell power to ensure emphatic crashes in Bitcoin and raking coins back near the bottoms. Although in reality, they wouldn't have to exercise any selling power to crash any hyperbolic market. That shit would happen all by itself without any assistance sooner or later. Wash, rinse repeat.