The "R" in URL is a misnomer. Designers of the early internet really wanted internet to be made of documents. It turned out the internet is actually made of interactive apps and services. Hence the impedance mismatch between terms and their practical meaning. URL was supposed to be used to locate resources in the resouce-oriented internet - instead, in the service-oriented internet, URL is a language of sorts for making remote procedure calls, some of which are read-only operations returning resources. So URLs are only as stable as the RPCs powering the service behind it. As services evolve, so do URLs.