Earlier quoted context omitted.
Is that a sensical definition of waste here? The difference in externalities has to be greater than and opposite to the delta in cost, in order for their decision to be wrong, I think. Just because externalities are ignored doesn't prove anything in particular if you don't know the relative change.
Fair point. Let's say, "Sure to be wasteful compared with a decision that takes everything into account." Or, more simply, "Sure to be more wasteful." Although for the people who experience the negative exernalities, by their account it will always be wasteful, in that they only experience costs, not gains.
That's a general fact about the world, that making decisions on incomplete information does not necessarily produce worse results. Sometimes things turn out better than you expected and sometimes worse.
People are often significantly biased, but that's separate from the issue of not taking "everything" into account.
It also seems obviously wrong to say that a negative externality is or should be considered in isolation. It's just silly to say that people who experience negative things only experience negative things or that any category of things that includes negative things must be net negative.