Suppose the central bank controls 51% of hashing power and wants to achieve a stimulus equivalent to a 3% increase in inflation.
Counter 1: The central banks of the world are honestly too stupid and short sighted to compete with the worlds largest super computer - Bitcoin.
Counter 2: The bank would not be able to use their 51% computer on a new chain using a different algorithm. We could have almost identical blocks on each chain and a hundred different chains/algorithms - they would not be able to keep up.
Final counter 3: Block IPs not trusted; all central blocks would then go into thin air - ONE update.