The bond market is a pretty strong check on government-backed currencies. It's a stretch to say that a central bank can just print money as it wishes.

Wait, why can't that miner simply unilaterally switch to a better algorithm? Especially if the switchover isn't drastic, the ledger could be effectively destroyed before the minority notices in time to do a clean fork.

For the fork to mean anything other clients have to also switch algorithms (or they won't even accept any of the fork blocks as valid).