RFC 3678: Why should final rule out downstream use cases that library authors never anticipated?

RFC 3678: Why should final rule out downstream use cases that library authors never anticipated? I'd like to raise a concern about RFC 3678 — Final Trait Methods. My concern is simple: Why should a library author's inability to anticipate a future use case become a hard compiler restriction for everyone else? This is an everyday reality of open-source development. Many libraries start as small projects built for their authors' own needs. Some eventually become widely adopted, while others re...