The United States government hates currencies competing with its own, so it may at some point have great motivation to destroy bitcoin.
Answers to the first question show that the government couldn't destroy bitcoin by cornering the market. Answers to the second question show that it would be very expensive for an individual to sustain a 51% attack.
However, the top comment on the referenced answer reads:
A related point is: why bother mounting such an attack? If you were indeed successful and started building your own block chain, then confidence in Bitcoin would plummet and investors would sell out their positions (assuming their positions could be sold out). The price would drop through the floor and the value of your exploit would be reduced to nothing.
Given the upvotes on this comment, many appear to agree that a sustained 51% would plummet bitcoin values. This would lead to extreme distrust, and possible collapse, of the system.
But $16.35M + $84k / day is likely spare change for some branches of the U.S. government. If the U.S. sustained a 51% attack almost indefinitely, would this destroy bitcoin? For them, the value of their exploit would hardly "be reduced to nothing", instead it could possibly save their inflationary currency.
One might even say the 51% attack investment would pay itself off with the amount of inflationary income they could generate given the downfall of bitcoin.
This same scenario may apply to any wealthy organization. One could bet on bitcoin falling and then apply this attack for profit. The IMF or a different government could have reason to attack Bitcoin.