Why must we compromise the democratic liberties of PoW for the security of PoS? Don't close your mind to the possibility that you may have overlooked better solutions to the 51% attack. To dismiss PoW out of fear of a 51% attack is to end the experiment of Bitcoin.
There is nothing "democratic" about proof of work. If i am rich enough to buy 10% of the hardware I get 10% of the "vote". If you are so poor you can't buy 0.1% of the hardware you don't even get 0.1% of the vote. I doubt proof of stake enhancement is possible in Bitcoin (difficult to make even modest changes) but they both use invested wealth to ensure the dominant chain is supported by the majority of the invested wealth (wealth with conviction).
Not much difference between:
a) I take $100K and buy 1% of the network hashing power
b) I take $100K and buy a combination of (less) hashing power and large enough stake to have 1% of the effective (stake weighted) hashing power
In both cases I am putting $100K at risk and gaining 1% of the network revenue and having 1% of the "vote". To think one is the Roman Republic and the other is serfdom is naive.
Well, forgive me for looking too far to the future, but this opens up the possibility of things like white-listing accounts. Haven't registered your bitcoin address with the Bitcoin Regulatory Commission? Then this monopoly is not approving your transaction. So not only is decentralization gone, but so is pseudonymity. You claim there is no credible mechanism, but the only basis I see for that is because you haven't thought one up. If you want to attack my idea, have at it. I haven't gone far in fleshing it out, but it certainly is a lot simpler than proof-of-stake so problems should be easier to bring to light.
Proof of stake doesn't necessarily mean monopoly or cartel. Proof of work is no guarantee that there won't be a monopoly or cartel.
If tomorrow Deepbit, Slush, and BTC Guild decided they would only build on their own blocks they would have 100% of the network doubling their revenue overnight. Since they no longer need any more hashing power they could close the doors to new hashing power and new miners. If the merged they could gain higher profits by having members turn off hashing power (but having it be hot idle under control of the cartel). They could operate a modest 1TH/s or so with the ability to engage up to 5TH/s+ if someone tried to break the cartel. Members could be issued shares and paid dividends from this entity. In time hardware could be moved to warehouses controlled by the cartel and replaced with more efficient equipment. As long as they have the ability to deploy more hashing power than any "upstart" they could leave most of it idle further boosting profits. Their idle hashing power is in effect an indirect form of proof of stake. On a long enough timeline they would form the basis for the "Bitcoin Regulatory Commission" you fear. Eventually datacenters full of ASIC processors providing tens of TH/s of on demand hashing power would be the method of control. As long as their potential hashing power is large enough there is no need to keep it online. Far more efficient to have it in "reserve" (thus producing no cost) and monitor the network. Members would simply be shareholders (with shares openly traded anyone could buy a stake) and Bitcoin would be a controlled centralized network.
I am not saying the top 3 WILL do that but the belief that somehow PoW is democratic, free, and open and PoS is closed, centralized, and monopolistic is just naive.