In my opinion, pete's formulation (no no-one really knows the full extent ...) is superior to Nigel's (No-one really knows the whether ...)

To begin with, pete's includes zero, so it subsumes the 'whether'. Then it is has a numerical value and so gives more information, whereas 'whether' is Boolean and so is less informative (in the information-theoretic sense.)

SO I don't think it can be classed as a false premise.

More legitimate would be to object to the framing. Pete's formulation refers to the full extent of the damage: it would be possible to reframe this as "no-one know the full extent of the benefits", which gives the same information, just changing the sign.

However, there are good reasons to use the 'damage' formulation, because it is standard business practice to do so: there is plenty you can read up online about risk assessment. But in short, the technique is to think about something and list all the problems it could have. Then alongside each of those you list what the impact if it occurs would be, an estimate of how probable it is and what you are doing to mitigate the risk.

All standard business practice. It doesn't matter how often people shout "Project Fear! Project Fear!", that is the normal method used throughout businesses everywhere. Nor do you wait until everything is over before carrying out the analysis; it is generally a business requirement to do it before attempting any change.

Pete's phraseology is in accord with that standard practice.

I have looked long and hard for any sign such an analysis has been done. Occasionally you get a leak to suggest something along those lines has been at least partially done, but it invariably dismissed as 'a working paper' or some such.