I now see a much more urgent and catastrophic threat vector potentially circa 2016.
Difficulty will tend to rise and fall such that the system-wide cost of mining is nearly the value of the coins mined (not perfectly but on a smoothed basis roughly true).
Thus the lowest-cost marginal miners will earn the highest profits and rates-of-return. Thus, as Satoshi Nakamoto anticipated, mining will become centralized among those with more capital who can buy the highest performance hardware.
This is more accentuated by the designed volatility in the price of Bitcoins, because a) marginal cost producers can go through periods of negative profits or no production, and b) more capital means more inertial reserve, economy-of-scale access to capital funding markets, and access to debt to delay insolvency so as to smooth volatility in market conditions.
EDIT: Relevant excerpt from linked question above which was deleted:
since Bitcoin is such a minuscule portion of the economy and can't create such global "network effects" on real productivity, appreciation in FX price is the main mechanism for evidence of the creation of value. Appreciation in price does create some value because it encourages the teaching, investment in, and thus spread of the currency. However, when the appreciation in FX price moves exponentially faster than any human calculable time-preference for opportunity cost, and exponentially faster than growth in spending for goods & services can (due to its connection to real world of profit & loss that adapts more slowly than speculation), then the price is dominated by wild guessing, i.e. emotional speculation.
So to capture a majority or maybe even most of the mining, an entity probably merely needs to subsidize on a large-scale.
The government or powerful elite has the funds to do this, either in public view using taxes (or deposit insurance on P2P currencies) or in documented black budgets.
Is there a scenario where the public would clamor for the government to fix a problem by doing this?
Imagine the public became heavily invested in wild "dot.com 2.0" run of Bitcoin price appreciation to say $1000, $10000, or $100000 (as some people expect). All bubbles eventually crash. Tangentially, the creator of Bitcoin would probably be a $billionaire.
So with an exponential crash in price, there could be a drastic reduction in miners, joined with a public wanting to blame something or someone. The government could probably find massive support for deposit insurance.
Thus I conclude that the computational Proof-of-Work design of Bitcoin is a serious threat to anyone who views Bitcoin as being independent of the control of the government. Noting that correlation isn't proof yet via Occam's Razor it is ostensibly designed for this outcome and thus to fall into the lap of the government.
And we are not talking about 2040. The pricing froth is building now so this could happen soon. And we also have another halving of the debasement payout rate in late 2016.
Which Btw, is when the repeating-throughout-all-recorded-history Economic Confidence Model expects this current bounce of capital flows into the USA dollar economy to turn hard down again a la 2008— exactly when we will need a P2P currency to help us avoid chaos of capital controls and liquidity collapse. Bitcoin could fail at precisely the time when we will need it most. That is what concerns me so much.
I would very, very much appreciate if any downvoters sign with a comment, in case this answer becomes prophetic we will be able to know how prescient you were.