What would "cost" more, the cost of printing enough fiat currency to buy > 51% of "POS Bitcoin" coins, or the cost of printing enough currency to have > 51% hashing power in the Bitcoin network?
You don't need some hypothetical scenario that involves printing money, we have actual cases that happened in shitcoins that have PoS algorithm in the past like the case with Steem network where exchanges took over the chain simply because they held a large number of that shitcoin without spending a dime.
https://bitcointalk.org/index.php?topic=5387588.msg59383033#msg59383033That's merely a "young, nacsent" altcoin. I was thinking about a more developed/mature coin, with high market value. As an example, Ethereum, which is going to make the switch to POS.
It makes no difference; it's a general, inherent problem of PoS. It's just as possible on the 'latest and greatest generation X' PoS blockchain, as it is on a 'young / nascent' or a 'developed / mature' PoS blockchain.
The very principle of PoS that the voting power is determined by the amount of coins you hold
is the issue at hand.
If the world's central banks printed enough fiat currency to buy 51% of the coins, that would imply they are buying at least 10 million coins. Are there that much for sale on the open markets? Attempting to buy it in a short period of time would drive demand over existing supply. We'd see $10m BTC sooner.
What if they will only buy it during bear cycles, and they're willing to take their time?
It's one option; or they might buy it off-chain (buying keys / lending the coins / keys). You can't notice that on-chain and it won't cause price fluctuations.