I think you're basing this on a false premise. There's no rule that states that every life form that ever existed must have any descendants. In fact the opposite is true, the vast majority of life forms were exctinct.

Famous case in point are the Ediacarean fossils in Newfoundland, the first known multicellular organisms. No descendants live today, they are completely extinct. Without these fossils we wouldn't even be aware of this branch of the evolutionary tree.