$3000 million is certainly a lot less than the cost of a military powerful enough to defend such a centralized system from government takeover. Also, I think the right comparison is not a hypothetical single server handling transactions, but the current bloated global financial industry. I'll bet $3000 million doesn't even cover the air conditioning bills for the offices of the financial institutions whose jobs would be better handled by Bitcoin.
As for the military: That may well be true, but who is willing to spend that kind of money in a sustained fashion just to have a system that is safe from government takeover? Libertarian ideology only gets you so far. Will libertarians put their money where their mouth is?
As for the bloated global financial industry, there are two points. First, it does more than just a simple ledger (think the overhead of dealing with fraud in a way that people are willing to accept). Second, it's plausible that Bitcoin will help de-bloat the financial industry, at least the payment services part, and that would certainly be welcome. That doesn't mean that Bitcoin will survive in the long run, though.
