Well, as a first point, OP's information is wrong. While it's true that the half-life of U235 is ~700 million years, it isn't true that half of the initial mass is lost. When U235 fissions, about 200 MeV of the initial mass is converted to energy. The total initial mass is about a thousand times that, though (E=mc^2). So, after 700 million years, assuming you kept it from falling apart, the initial 15 lb lump would only lose ~1/4 of an ounce as energy.
I didn't make this meme but oops
You are correct in that there is only 7.5lb of uranium left, it's just that there is also a bunch of other stuff that used to be uranium too.
Well, as a first point, OP's information is wrong. While it's true that the half-life of U235 is ~700 million years, it isn't true that half of the initial mass is lost. When U235 fissions, about 200 MeV of the initial mass is converted to energy. The total initial mass is about a thousand times that, though (E=mc^2). So, after 700 million years, assuming you kept it from falling apart, the initial 15 lb lump would only lose ~1/4 of an ounce as energy.
I didn't make this meme but oops
You are correct in that there is only 7.5lb of uranium left, it's just that there is also a bunch of other stuff that used to be uranium too.