Actually, Ripple is somewhat decentralized, but is prone to censorship because new nodes generally have a set Unique Node List (UNL) that is controlled by ripple labs. However, node operators can change their trusted nodes at any time if they wish, so ripple has a chance to be much more decentralized as time goes on if a diverse set of nodes and validators emerge.
Besides, the bulk of security comes from digital signatures (which ripple has) and the constant ability for users to hard fork the system through open source code (which ripple has). So far, I am not aware of any major attack on ripple consensus, so the naysayers have some proving to do.
Contrary to popular belief, some fiat currencies are somewhat decentralized. For example, monetary policy for the US dollar is set by the federal reserve, the head of which is elected by the president, who is elected by the populous. However, the dollar has suffered massive attack (bailouts, qe, inflation) primarily because users don't have digital signatures and they can't fork at will. Imagine if a faction of US dollar users could have forked the US dollar at the point of the first bailout in 2008. I would have traded all bailout dollars for real dollars myself.
Thank you helikopterben for this information regarding Ripple.
Always appreciate learning more about cryptocurrency and the money system in general.
Cheers.
News out today about Ripple and AMEX:
https://ripple.com/insights/american-express-joins-ripplenet-giving-visibility-and-speed-to-global-commercial-payments/