It is not the job of OSM to enforce law. We provide information, what people do with it is their decision. There are many reasons to keep things in memory: Why does this path have such a weird shape? Why does this path not continue? Must be a mistake... oh there once was a way, now it makes sense, I don't have to investigate that. Also you see that parts of the deleted ways in other changesets have already been remapped, so they are obviously of interest. The remapping will continue anyways but the work could be used in a better way than on ways that have already been drawn earlier.