(because that credit is a centralized monetary mechanism).
I promise you it isn't. I can extend credit in any currency I have to anyone I choose.
It may seem this way because the money you happen to use is printed by one special organization, but that is not a property of all money. And even with central money I can extend credit up to the amount I have.
I don't understand what you mean?! You can have all sorts of credit (like with your neighbor), but what does that buy for most people? The vast majority of the world's money supply is created by monopolistic state law, which in turn means by monopolistic credit entities.
For BitCoin it makes no sense to have a few entities generating credit out of thin air, when BitCoins can be directly generated out of thin air by individuals (well, it's indirectly from electricity). However, such entities would start popping up and create credit if the BitCoin supply remains limited.
I mean, then people could just use Ripple and be done with the whole concept of medium of exchange.