Here [1] is a picture of the temporary storage at the Yankee nuclear power plant temporary storage (sorry I could not find better pictures from above to give more accurate size). There are ~90 commercial nuclear reactor in the US. I can immediately see that this would not fit into a football field sized area. And this is for a single country and ignores waste that was already transported away. And no things will not decay to harmlessness during the lifetime of a plant. They decay to a level where the containers are not very warm anymore so they can be more easily transported. Also low level waste is not harmless either, and there's going to be a lot of it when decommissioning a plant.