By the same token, you could have exactly the same fear about bitcoin. Its ownership is concentrated in relatively few hands so no attack is necessary to mess up its prices (though it's in the interests of the whales not to do so). If there was the motivation to do so, governments around the world could certainly cooperate to shut down bitcoin (at least temporarily). They could commandeer major mining computers and force them to stop adding transactions to the block chain. Of course, that won't happen because cryptocurrency is more useful than brown paper bags for politicians as well as criminals.
Similarly, Ripple Labs isn't going to sabotage Ripple either. Major banks are using Ripple as a cheaper and faster alternative to SWIFT and they wouldn't do that if they had any concerns about the transactions on the Ripple network. Ripple Labs complies fully with KYC laws so there is no problem there either.
The most important technical aspect of Stellar (if you are still scared of Ripple) is that it uses consensus rather than competition to add transactions to the block chain. This uses far less energy than bitcoin and points the way forward for decentralized transactions.