The death-knell in my opinion for true decentralization of the bitcoin network (ignoring mining for a moment) happened when the blockchain became too large for it to be practical for normal users to have the whole blockchain on their machine and operate a real node. I am sure there is some creative solution to this, perhaps an additional mining process by which old records are combined to save space (i.e. transactions older than a week get squashed so that only the resulting balance is stored instead of the individual transactions, perhaps). It might also help to not even store information about a wallet address on the blockchain unless it has a nonzero balance. I have no idea how this is actually implemented so this is all speculation.