There's an upper limit of 21 million Bitcoins. Currently that approx. 100 million USD. For big players like Goldman Sachs that's peanuts.
Should they decide one day that Bitcoins might impose a threat on them in the near future, they could buy large amounts of bitscoins - and "destroy" them (e.g. move them to offline wallets - and "throw away the key").
Is that a valid scenario? What do you think?
No. I think you have absolutely no idea how the market works. Not all bitcoins are for sale at the same price, and some are not for sale at all. If a person (or corporation or government) wants to buy a large number of bitcoins, they can only do so if they can convince a large number of people to sell, and some people will not be convinced to sell their bitcoins unless a high price is offered. For this reason, the more bitcoins someone wants to buy, the higher the price per bitcoin becomes, and they won't be able to buy all the bitcoins because some people will not be willing to sell for any price.
If someone did
buy a large quantity of bitcoins, driving up the price in the process, and then sold them again, the price would just drop back down for the same reason (to convince enough people to buy, they need to offer a low price). If they destroyed their bitcoins instead of reselling them, the price would simply stay high forever. They lose all the money they put in and everyone else becomes a little bit richer. For some reason, I just don't see any bank or government doing something like this.