I'm not sure if allowing default implementations in interfaces is the way to go here.
To me it looks like a workaround / hack for non existing multi inheritance.
Why not either make multi inheritance possible instead or allow traits with interfaces as suggested by Victor?
Multi-inheritance seems to be the hot topic that prevented this RFC from being approved even though it was not the RFC target. Multi-inheritance is an afterthought that may or may not be abused with this change. What we want would actually be just the convenience of doing what Traits already allow while reducing potential BC break impact coming from interface changes. There are interfaces originated from the interface segregation mindset that often has only 1 implementation and could very well take advantage of default implementation for a simpler system design.
We spend a lot more time reading code than writing it. The elegance of short closure combined with the convenience of variable scope usage has already shown to be a game changer on Typescript and there doesn’t seem to be any technical issue with having it on PHP.