First of all, it would require a lot of money to invest in equipment. Orders like this are hard to keep secret. Someone would investigate. It would have to be a sustained 51% for many many blocks or else the miner would be DDOSd, IP blacklisted, or worse. That would end their Bitcoin career, and if they are lucky they could sell their used equipment to partially recoup their millions. That equipment would then make Bitcoin stronger against the next attack. If they were successful for any length of time, the price of Bitcoin would plummet (markets would close) and they would not be able to sell their fake bitcoins anyway. Besides, the people with that much money to throw away are unconcerned about Bitcoin anyway because they don't care about anyone's future but themselves.

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Everyone knows that Bitcoin mining MUST become more centralized (in order to scale up transactions) and thus it will likely (almost certainly IMO) eventually become controlled by the G20 that can regulate a few 100s of mining nodes. Will be justified by the G20 doing coordination against terrorism, money laundering, and tax cheating.

And the masses who use Coinbase wallets and other large providers such as the Blockstream.com (3 million wallets) CEO shaking hands with the Prime Minister of the UK upthread, won't care! They are sheep. They follow. They are preoccupied.