The AuthorityIDs are global in scope. Even if the resolution of a particular URN no longer makes any sense, it is easy to fully resolve the URN to something that is readable by the user, and to do this forever. If a site gets too busy, mirror sites can be specified using standard DNS procedures. All parts of the specification are designed to be extensible. Additional resolution requests and resolution results can be defined, and so on.
All domain names are already independent.
The resolver uses a simple HTTP exchange, supported by dozens of browsers and servers today. The DNS naming authorities are scalable without any notification to, or approval from, a central authority. The character set for URNs is simple and small, the same as URLs. Security features that are added to HTTP will probably increase the security of resolving URNs.