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.
Good point; however this only works on coins with limited market cap; many PoS coins are unlimited, so they can buy bulks of it without problem.