I have not read the book and I am just curious as to how the movie could have been improved upon?
I don’t think it can. The book is really interesting, but it’s done as oral histories and lacks a single narrative thread. Clearly (from the trailers) they already broke away from that for the movie. But it could still be a good movie, and some of the interesting parts of the book (how the response was organized, the argument of actually abandoning large parts of the country/ world, how desk job American had to adjust to a new world where their skills didn’t matter) could still be in it and would be fun.