If you look at the responses of individuals directly associated with the atomic project itself (I'm reading a biography of J. Robert Oppenheimer, presently), you'll be well aware that the concerns many addressed were not merely of the immediate impacts of the weapon, but of future developments, which would include a tremendous reduction in costs for delivering destruction on an absolutely unprecedented scale.

Costs matter. See the Jevons Paradox. And, as I've said before, scale matters (https://news.ycombinator.com/item?id=13542735#13548498) (and more: https://hn.algolia.com/?query=dredmorbius%20"scale%20matters...).

It matters more than very nearly anything else.