I mean stolen because of no chargeback. Anyone could 'sell' you something and not send it once they're paid. (If people new to BTC believe it's completely secure they may buy from randoms and get defrauded)
In regard the 51%, I know there is an agreement that nobody goes over 39.99% but if somebody was to get an investor to buy the computer power who wanted to charge fees there's no law from stopping them doing so. Or if a bank decided to go into the bitcoin business. I'm sure a bank could afford computers to do this.
Although I think another problem with Bitcoin is the fact that an investment bank could buy up a huge amount of bitcoin if they though it was a threat. I think there's about $7billion of BTC in circulation. I think it was Goldman Sachs that did this with raw aluminium, pumping the price up by buying HUGE amounts making it rare then sold it all bringing the price crashing down again.
1. The chargeback issue is easily remedied by using an escrow service (either automated or otherwise)...look at OpenBazaar for example...this is really a none issue....you're also again assigning responsibility to a technology for the failure of some to use it properly or understand it properly. That doesn't change what it is and can do that never could be done before.
2. The amount of money required to buy enough hash power to control the network for a few minutes is ENORMOUS and the gain would be minuscule at best. This is again a non-issue.
3. I'm not sure why your last point about a large bank buying up available Bitcoin supply and sending prices to the moon is somehow a problem? People would just transact in milibits or satoshis...it makes no difference to the technology itself. If you're thinking of it only as an investment, then perhaps...but the one thing they can't do is print more Bitcoins or to paperize more Bitcoins than there are...which is what they currently do with Gold....
peace