Nice discussion going on here so let me chime in a bit:

Before you can call something decentralized or centralized you must first define what these terms mean in a concrete and objective manner.  I will define them as follows:

1) You are fully decentralized when you are fully in control of your own assets.  The free market and voluntary trade of physical goods is full decentralization. 

2) When fractional ownership of a common good is in play, then you are decentralized when your control/influence is proportional to your own assets.


Mining can never be decentralized because the miners are in control over the shareholders assets and control/ownership are completely decoupled.  Sure you can sign a transaction, but it must be 'approved' by the miners.   


I like Nxt and their transparent forging approach is the closest thing out there to being truly decentralized.   Unfortunately, because only a subset of the users forge and there is no restriction in how concentrated forging power can be.   

I think that this entire debate of mining vs POS is founded upon a lack of understanding on what Bitcoin really is.  It has not solved the Byzantine Generals Problem... it just defined a rule set for consensus that allows anyone with money to control the network and once controlled there are no alternatives.

Likewise, CPU only POW merely means the government will certainly control it given the percentage of the population employed by the government and government-friendly corporations and that each of these employees has a computer.   Mere economic analysis is all that you need to understand that POW is the ultimate means to assure centralization because it erects barriers to entry that are insurmountable by everyone wanting fork as a result of the miners taking the chain in the wrong direction.

Because of a false belief about the nature of consensus many seem to think that 'Zero-Trust' models are the only ones that are valid.   I contend you are always trusting something or someone.  Random selection of anonymous individuals to produce blocks is both slower and allows 'random' bad behavior on a temporary basis.  It is less reliable.   

DPOS is all about creating a system that gives people financial incentive to do the right thing and maintains the ability to 'fire them' quickly if they don't with 100% certainty of getting caught.   The delegates have no power *BUT* to do the right thing.   

Can they collude?  Assuming they did collude the only thing they could do is block transactions and it would be instantly detected by all peers.   A fork would be launched immediately with these delegates stripped and the two chains would compete against each other in the market.  Assuming the delegates were just random people that attempted to screw with the system then consensus would be maintained.