1) It's not too expensive if you are a large agency. If the goal is to destroy bitcoin because of its potential future value, then this may be an efficient attack. The meaning of the attack is to prove that the blockchain is malleable and reduce its standing as a stable useful currency.
If you are a large government agency, no sane person in the government would allow you to do such a thing. It would be a massive waste of money and after the whole thing is done, what would they do with the leftover equipment? It can't be used for anything else.
2) Once devs have to modify code in the core client to make a decision (no matter how clear it is that one party is "good" and the other is "evil"), we have centralized and given the power to choose which chain dominates to the devs.
And of course that has already been done with checkpoints. We are letting the devs choose which chain we use because of the checkpoints already in place.
qwk thank you for the link, checkpoint blocks are an interesting concept
It is not just a concept, the checkpoints already exist.
The code is at
https://github.com/bitcoin/bitcoin/blob/master/src/chainparams.cpp#L119Here are the current checkpoints:
checkpointData = (CCheckpointData) {
boost::assign::map_list_of
( 11111, uint256S("0x0000000069e244f73d78e8fd29ba2fd2ed618bd6fa2ee92559f542fdb26e7c1d"))
( 33333, uint256S("0x000000002dd5588a74784eaa7ab0507a18ad16a236e7b1ce69f00d7ddfb5d0a6"))
( 74000, uint256S("0x0000000000573993a3c9e41ce34471c079dcf5f52a0e824a81e7f953b8661a20"))
(105000, uint256S("0x00000000000291ce28027faea320c8d2b054b2e0fe44a773f3eefb151d6bdc97"))
(134444, uint256S("0x00000000000005b12ffd4cd315cd34ffd4a594f430ac814c91184a0d42d2b0fe"))
(168000, uint256S("0x000000000000099e61ea72015e79632f216fe6cb33d7899acb35b75c8303b763"))
(193000, uint256S("0x000000000000059f452a5f7340de6682a977387c17010ff6e6c3bd83ca8b1317"))
(210000, uint256S("0x000000000000048b95347e83192f69cf0366076336c639f9b7228e9ba171342e"))
(216116, uint256S("0x00000000000001b4f4b433e81ee46494af945cf96014816a4e2370f11b23df4e"))
(225430, uint256S("0x00000000000001c108384350f74090433e7fcf79a606b8e797f065b130575932"))
(250000, uint256S("0x000000000000003887df1f29024b06fc2200b55f8af8f35453d7be294df2d214"))
(279000, uint256S("0x0000000000000001ae8c72a0b0c301f67e3afca10e819efa9041e458e9bd7e40"))
(295000, uint256S("0x00000000000000004d9b4ef50f0f9d686fd69db2e03af35a100370c64632a983")),
1397080064, // * UNIX timestamp of last checkpoint block
36544669, // * total number of transactions between genesis and last checkpoint
// (the tx=... number in the SetBestChain debug.log lines)
60000.0 // * estimated number of transactions per day after checkpoint
};