My original response was more thoughtful, but I felt that summed up my point better.

The question is not what's possible, but what's likely and how much it will cost. If the likelihood is low and the cost is small, then you cannot justify extravagant preparation.

An attack against civilian infrastructure is terrorism no matter who does it. It may be a singular event, part of a chain of attacks, or a prelude to a full-scale military offensive. It's still terrorism, because it terrorizes the civilian population.

Irrational fear should not dictate our budget priorities. We can certainly take some precautions, and develop contingency plans. But at the end of the day, if someone is capable and determined enough, it's going to happen, regardless of how many much money and how many contractors and consultants we throw at the problem. The best we can do is take it in stride and rebuild.

You continue to not really be thinking about my point, insisting on framing it in a pre-canned terrorism argument. I am explicitly talking about high-cost events, and at best you can call the probability unknown, for things like a biological attack or a computer attack that perhaps even accidentally ends up taking out some very important part of the infrastructure. We already have good reasons to be concerned that a high-cost biological attack could be launched out of someone's literal garage, and it is not an irrational fear; we have plausible plans on how it could be done. Indeed I'd argue that being unconcerned about it is irrational, and being unconcerned about it because so far "terrorists", a small subset of the group of people I'm concerned about, have only managed to "fly planes into a couple of buildings". It's basically a non-sequitur argument, because a great deal of the point I'm making is that is only a small part of the total interesting threat profile.

What is your point then, exactly?

Are you arguing that there are eventualities which could be disastrous, and they need not be the product of willful malice? Then of course I agree with you.

Is your point that we can and should take all possible precautions against such eventualities, regardless of cost? That's where I take disagreement. We are, after all, arguing in the context of a larger discussion here.

Our efforts at preparedness must be in proportion to the risk, which is derived from both the potential cost and our best estimate of the likelihood. Bad things happen; it's terrible, it sucks, but it's unavoidable. We can and will bankrupt ourselves trying to swat every fly. It doesn't matter that a disaster could be catastrophic if we create a catastrophe by trying to avert every disaster.

"We already have good reasons to be concerned that a high-cost biological attack could be launched out of someone's literal garage, and it is not an irrational fear."

It's easy for people to just pick up a knife at a restaurant and stab someone too. People don't spend an inordinate amount of time worrying about this though because it just doesn't happen that often.

But the problem is that it ends up being a self-fulfilled prophecy.

People without any part in the original conflict is getting affected by it and then suddenly they are part of it.

That is the root of problems here.