> Does anyone say "we don't know if Einstein could do this because we were really close or because he was really smart?"

It turns out my reading is somewhat topical. I've been reading Rhodes' "The Making of the Atomic Bomb" and of the things he takes great pains to argue (I was not quite anticipating how much I'd be trying to recall my high school science classes to make sense of his account of various experiments) is that the development toward the atomic bomb was more or less inexorable and if at any point someone said "this is too far; let's stop here" there would be others to take his place. So, maybe, to answer your question.

It’s been a while since I read it, but I recall Rhodes’ point being that once the fundamentals of fission in heavy elements were validated, making a working bomb was no longer primarily a question of science, but one of engineering.