Even though oil runs out, it does have the advantage of carrying away contaminants. Likewise grease needs to be changed periodically, for the same reason. Maybe not so important with good running surfaces, but more important to prevent further wear, once it starts.
I recall some ‘wear-rate trials’, carried out to compare cheap versus expensive grinding media. The ‘cheap’ stuff apparently came out better value because the expensive materials produced hard carbides which acted as a grinding medium with the more expensive material. Proper tests, where the wear-products were removed, as the test proceeded, resulted in far superior results for the more expensive product.