Government intervention is the bigger concern right now, IMO (and no, I'm not suggesting any government is anywhere close to looking at BTC as a problem yet).
If you could build a GH/s machine for even $1000, you could recreate the current power of the network for under $5M with simple consumer hardware.
That is pocket change to the US Gov.
Add in labor, DC build out, NOC, etc and you could build a 5TH/s+ BTC mining DC for easily $10-20M to break the 50% barrier. It's fine to say stuff like "the top 500 supercomputers don't equal the processing power of the current network" but 5000x 1 gigahash machines do. I guarantee you with a $20M "btc-killing" budget a company could get it done. This is certainly not a single-purpose datacenter either and has a lot of potential value for defense in general.
Hopefully by the point world governments take notice the network is already in double digit TH/s range, but right now it's not as invincible as people seem to suggest.
How comfortable would you be trading stuff for BTC if you can't get any confirmations that they money wasn't double-spent?
I'd think international banking cartels would be more of a threat. But regardless, let's assume there is some
threat entity [hereafter: "TE"] that would try to shut it down. The question is: Is there even a viable possible catastrophic threat to bitcoin(?) Even if TE could swamp the network down and suck up all the remaining coins, there would still be 30% of all coins ever to be released already in circulation. TE would be holding, let's assume for argument, 70% of all bitcoins. So, what does TE do with them? Dump them on the market? Ok, then what did TE gain? What if TE hordes them and destroys them? Ok, bitcoins value go up due to lower supply... but bitcoins are divisible up to 8 decimal places. So, instead of there being a possible 2.1E10^15 coin fractions, there are then 0.7E10^15 or 7.0E10^14. Let's see, that's 700 Trillion coin division units. Assume a worldwide population of 10billion, and that leaves 70,000 units per person to spend. Of course, this is an extreme scenario and discounts that babies & very young children might not need to be counted, but it shows that there are sufficient increments remaining to perform as a mechanism of exchange fairly easily for all people - I think.
However, perhaps the threat possible is not related to hording or dumping...What is that threat? Hacking the bitcoin process? Maybe, it's just demotivation to any potentially new bitcoin miners or holders. How is bitcoin compromised? And more importantly, what are potential counters to that? Afterall, it is suppose to be designed to be extremely secure.