[B-Con]

If you add a method to an interface, you break every source file that uses a concrete type in place of the interface (ie, passes a struct to a function that takes an interface) unless you also update all the concrete types to implement the new method (or you update them embed the interface, which is yucky).

For a public interface, you have to track down all the clients, which may be infeasible, especially in an open ecosystem.