abolishing all labor law would vastly increase the size of the economy

[citation needed], as the saying goes.

I kind of doubt it. There are virtually no serious non-Marxist economists who believe that artificially raising the cost of labor, capital, or any other economic input diminishes output. The real debate is over whether it is appropriate to do so for other reasons, like fairness, justice, equality, and so on. So, if you really need a citation, I'd say that any first-year economics textbook would do it.

it would probably be a mistake for the list to pick specific economic policies on the basis that they produce the fastest economic growth, since then the discussion would be in danger of being politicized

My main point was about the mental process that generated the list. It a constraint on the process that generates the list is that it must be 100% de-politicized, then I wouldn't put much faith in the list. And it reads to me like it has been.

fastest economic growth should not be the only criterion, unless that really is the only thing that influences robustness

Sure, but that main point again was about the process that led to the list. There's a cost to everything on the list. Just because taxes pay for some item on it doesn't make it free, even in terms of robustness.

As I understand it, RyanCarey is interested in threats to human civilization as a whole rather than to individual human civilizations. Human civilization as a whole doesn't have laws, regulations, taxation, etc. If one nation collapses under the weight of its own regulatory burden then others will presumably take note.

One would hope, but they seem to be moving together for the past 20 years or so toward greater regulation, and hence greater fragility. If you can point me to a country whose published laws and regulations are shorter now than they were 10 years ago, I will happily retract the point (and consider buying a second home there).

And I would say that there are some legal jurisdictions that, if they failed quickly enough, could bring down the entirety of civilization. The US and EU are the two that come to mind. Two EMP devices, or two large enough asteroids, might do it.

How widely held, and how well supported, is the theory that the Roman empire failed because of overregulation and overtaxation?

It was the orthodox explanation in my economic history class that I took in 1988. I sometimes return to the subject to see if anyone has overturned that theory, and have never seen anything along those lines. The regulation was mainly driven by taxation,. The state raised revenues to the point that avoidance was really problematic, then instituted heavy controls on individuals to ensure payment. For example, in the late empire, most taxes were levied in kind; the regulatory response was that the law positively required the eldest son to succeed to his father's profession, property, and station -- so that the authorities could ensure that they were getting the right in-kind taxation. Some Roman citizens abandoned their property in order to avoid taxation, like runaway slaves. There is a theory that this is where serfdom originated, though I suspect the reality was more culturally mixed. The regulation had vast cost beyond the taxes being paid, because it prevented the movement of resources to more productive uses, either by changing jobs or by moving locations.

In any case, the point is that the regulatory structure created civilizational fragility. It didn't take much after that for Rome to fall. I mean seriously -- barbarian invaders? Rome had dealt with that for a thousand years and had always recovered from any reverses. It's like the signature of the Roman Republic that they lost battles, won wars, and came back stronger than before. But the empire became a different thing.