Exactly this. You can't hype up an audience to that degree and make them sit through 3 hours without showing them something theyve never seen before. And at this point you can't just Nolan your way out by making the blast larger than life, because I think by now we possibly all imagine it bigger than it actually was. But what you can do is some sort of trippy shit where the blast goes on for ages, where it ripping things apart is shown in some sort of artistic and novel way, perhaps something emphasising an old world is being torn apart and this is a new nuclear age. ANYTHING. except a half hearted blast that looked and felt like half hearted CG. And the irony is the production team went out of their way to avoid using CG. It was just immensely unsatisfying and rushed and all the momentum of the film was lost. It just didn't feel momentus enough.

If they'd been bolder they've have culminated with the bombing of Hiroshima and shown the horror in some new terrifying light..