Perhaps in that case the issue is that you are too much of a defeatist. You think, "well, all this crap will happen anyway, so let's just let it and figure out what to do when it does," while others thing, "all this crap has a damn good chance of happening, so let's figure out what we can do to prevent it in the first place." Very subtle difference.
Ah, but I am telling you what should be done. You just don't want to listen because it is unpleasant to hear.
I'm not promising that it will work against any possible opponent because I am not a snake-oil salesmen.
Consider BFL for a moment.
They claim to be able to construct "mini-rig SC" that has a 1.5 Terahash output. They plan to sell these at retail for USD $30,000. I would guess that the marginal cost of each unit is at most USD $10,000. Most of the expenses are all the R&D and prototyping that needs to be done to manufacture the first unit. Once those steps are complete manufacturing costs will be very low.
What does this imply? It means that after they have a working prototype, BFL will have a lot of power over bitcoin and strong incentives to expropriate bitcoin owners.
They could decide to pump out 20 of their units at a cost of about USD$200,000. That $200k will be enough to 51% the bitcoin network. Say they do this and use their power to implement a 1% tax on all txn inputs.
Now, you will say that doesn't matter. If bitcoin gets 51%'d then bitcoin's value will drop to 0 instantly. Are you sure, what if there is a 2% chance that value only drops to $5 and a 98% chance that it drops to 0.
Okay so now consider the scenario where bitcoins are worth $5. Total output volume is over 1 million BTC. So BFL could earn 10000 BTC a day doing this. And that is on top of the 3600 BTC a day they would earned through mining. With our new value of $5 that is USD$68,000 a day in revenue.
Say then keep this up for 1 year. Will they want to 51% bitcoin? Yes.
Their expected profit will be 0.98*(-200,000)+0.02*(-200,000+68,000*365)=USD$296400. That is a 150% expected ROI. Not bad. And this is assuming a 98% chance of complete and total loss.
Now, you will say that doesn't matter. BFL can earn more from selling rigs. I agree completely. But they don't have to pick one or the other. They can have their cake and eat their 51% attack too. They can sell $2 million dollars in rigs. If they do this, then they will be able to produce rigs en masse. As they expand production, marginal costs will fall certainly fall below $10k per rig. Let's say they drop to $5k per rig. Now BFL made a bunch of money from selling rigs. However the market is saturated. Mining is not profitable. What should BFL do to turn a profit now? They certainly can't sell any more rigs. A 51% attack of course.
If they have sold $2 million worth of rigs at retail, then the aggregate hashing power will grow to at most 125 Terahash. To take over, BFL will need to make 83 rigs for themselves to operate. At the reduced marginal cost, this will require USD$415,000 of investment. Use the scenario from above. Should BFL 51% the network?
Their expected profit will be 0.98*(-415,000)+0.02*(-415,000+68,000*365)=USD$81,400. A 19.6% ROI, not eye-popping but still worthwhile. And this is assuming a 98% chance of complete and total loss.
What do we learn from this? If we think there is even a very small probability that bitcoin will retain positive value after it gets 51%'d, then greedy hardware manufacturers should 51% it. Is it reasonable to think that bitcoin will retain positive value? Sure. You yourself said that 51% PoW attacks are reversible. If so, bitcoin price dropping to 0 will be a superb opportunity to buy up cheap coins for you. Think of it, USD$0.10 bitcoins. Wouldn't you be a fool not to buy them? Even if you don't won't someone else? A year later when the 51% issue is fixed, you could be a rich man. (Of course you will have helped BFL get rich in the process)
Well, if instead we use the scenario that a 51% attack causes bitcoin price to drop to USD$0.10 with certainty, all the numbers are exactly the same as before. Attacks are still profitable.
Maybe BFL are altruistic. Fine. Maybe they are. The convicted felon CEO who defrauded the elderly out of millions of dollars may have turned around. It is certainly possible. This isn't a one time thing though. Will the next hardware innovators be good guys to? What if they are a state-owned company in China? Do you know how amoral those guys are. They seize children from their poor families for sale on international adoption markets. No joke. Do you think they will bat an eye at expropriating bitcoin owners in the US? Do you think the US gov't will care if this happens. I sure don't. You should also keep in mind that as the block reward drops the profits from selling hardware on the open market will fall. The profits from 51% attack however will remain approximately fixed. That is not good. Not good at all.
That is why I suggest PoS. With PoS, the average coin holder makes a profit when a 51% attack occurs. That type of insurance is a very good thing. Also, attacks aren't possible without spending say USD$60 million and up. That's a good thing too. Yes, attacks can still happen. That's too bad. However, just because you can't fix everything does not mean you shouldn't try to fix as much as possible.
If someone has a better suggestion, I am all ears. However, I will not listen if someone says we can wait until the attack occurs to worry about the issue. That is ridiculous. You won't be able to fix something like this on the fly.