I agree that competitive mining has some advantages. However, in other respects monopoly looks superior. Just as importantly, mining looks like a natural monopoly. If monopolized mining is highly profitable, then bitcoin will likely become a monopoly. The system should be prepared to accommodate a monopolist. Proof-of-stake is a good way of ensuring that the monopolist will behave himself.
Can you explain how a monopolized Bitcoin is better than a central currency issuer? And why do we need the whole proof of work/stake thing, instead of the monopolist just synchronizing transactions in its internal database?
Transparent, psuedonymous, and irreversible txns. It is not clear how this can be maintained without an alternate currency system such as bitcoin. A company which owns the majority of bitcoins and maintains a centralized txn database is more trustworthy than a company which just maintains a centralized txn database.
What if it charges exorbitant fees of $0.3 + 3%?
The monopolist will have lower marginal costs because most of its hashing capacity could remain idle. Fees could go up or down. I think they would go down because the marginal cost savings are potentially extremely large, but it is an empirical question. Remember that implicit fees are currently outrageously high. Low fees should not to be a talking point when so much new money is being issued.
What if its servers are down and nobody can process transactions?
I think they would have strong incentives to keep their servers up. If they went down for a prolonged period, then private miners could take over. The monopolist would have strong incentives to recognize their txns.
What if it is attacked by some government agency?
Proof-of-work is just as easy to attack under centralization as it is under decentralization. If the monopolist is unable to maintain dominant hashing power, then competitive miners or another monopolist could take over. If the gov't decides to maintain dominant hashing power, then competitive miners would be fucked anyway. Same goes for proof-of-stake, if a private actor can silently acquire a majority stake, then a gov't could do so easily.
What if it goes bankrupt due to bad management?
If it ceases hashing, then competitive miners or another monopolist will take over. The blockchain will remain transparent and there will be a market for control over it.
What if it starts posing restrictions on how one can use their bitcoins?
If it does this voluntarily, it will lose a principal niche market for drugs, money laundering, etc. I don't think it would want to do this. If the government forced the monopolist to do this, then either a) another monopolist will take over who can operate more profitably in a jurisdiction without these constraints, or b) the gov't would act the same way against a p2p chain. In any case, it is not plausible for bitcoin to grow tremendously and simultaneously remain unregulated.