How exactly is Ripple decentralized when OpenCoin has so much XRP? Let us assume in a best case scenario, the Ripple system gets opensourced this year and the 50 billion XRP are distributed fairly to many people in the world.

One of the reasons of having XRP currency is to prevent spam and provide safety to the Ripple network from abusing.

OpenCoin has about 50 billion XRP or even 30 billion XRP without the founders count. So they can technically use their massive XRP to harm the system if they need to. I have not reviewed how many different and harmful ways one can abuse the Ripple network if they have too many ripples.

Why does any single entity needs to have such amount of power over the network? Technically, for us to trust Ripple, we need to trust the OpenCoin entity forever. Even if OpenCoin people are good, what if their accounts got hacked and the XRP is with malicious people?
There are only two known "attacks" you can perform with a large amount of XRP.

First, you can dump a large amount of XRP on the market. This will reduce the value of XRP, which OpenCoin would have to report as a loss. The net effect would be that people who hold XRP as a store of value would be harmed. So if you choose to use XRP as a store of value, you are incurring the risk that OpenCoin might hurt you by hurting itself more.

Second, you can sustain a large flow of transactions that would bid up the transaction fee on the network. This would mean that people who wanted to perform Ripple transactions would have to wait or would have to "outbid" OpenCoin for transaction slots. This would cost huge amounts of XRP for every minute you wanted to keep the network unusable. So this attack could just be waited out.

But really the premise behind this is false. OpenCoin's XRP has the same value regardless of how it was acquired. So worrying about this is really no different from worrying that someone with large amounts of Bitcoins might dump them on the market crashing the price. It's true, but people don't do that often because it hurts them more than it hurts anyone else. So, yes, it's a risk.

But generally speaking, there are many kinds of attacks someone can perform at their own expense to hurt you. Someone could flood the Bitcoin network with transactions with a .2 BTC transaction fee each, effectively knocking out Satoshi Dice. It's not rational to unduly worry about these kinds of things because it's so rare for someone to attack you in a way that hurts them more than you unless they have some clear motive to harm you and otherwise you'd pretty much have to worry about everything. And, of course, OpenCoin has no motive to deter adoption of the Ripple network.

(I'm an employee of OpenCoin speaking only for myself.)