What if an entity (a government, central bank, etc.) could maintain 51% of the hash power for an indefinite amount of time? Couldn't they repeatedly attack the chain? I find it hard to believe that sort of attack could be mitigated without switching algorithms.
In the unlikely event this happens(Governments cannot keep secrets well) within 1-3 transactions the community would notice what was happening and most people would temporarily freeze transactions for a few hours to a day (like how some exchanges stopped processing transactions during DDOS attacks or the transaction malleability scare) and than developers roll out a hard fork.
When the new code is rolled out to prevent this attack(multiple ways to block these 51% miners ) than every individual would decide whether to accept the changes or not and install the new bitcoin core code wallet implementation or any other implementation codebase(Libbitcoin, ect...another reason Bitcoin is more robust than any other coin). The users that don't upgrade would be continued to be vulnerable to the 51% attack (which would now technically be close to a 100% attack) and all other users and miners will have hard forked the coin unmolested by the attack with half the difficulty rate.
Under such scenario it is likely that the miners that attacked the network would now control a version of the blockchain that would quickly fall out of favor and decrease in value and thus essentially spending billions of dollars(The equipment alone is just part of the cost of the attack) to slightly tarnish Bitcoins reputation and create a few fake transactions on the network.