The need for this jumped out at me during Writergate. People had alot of trouble understanding exactly how all the pieces fit together, and there was no good place to document that. The documentation (or the code people went to to understand it) was always on an implementation. Having an interface would have given Zig a place to hang the Reader/Writer documentation and allowed a quick way for people to understand the expectations it places on implementations without further complications.
For Zig, I don't even want it to automatically handle the vtable like other languages...I'm comfortable with the way people implement different kinds of dynamic dispatch now. All I want is a type-level construct that describes what fields/functions a struct has and nothing else. No effect on runtime data or automatic upcasting or anything. Just a way to say "if this looks like this, it can be considered this type."
I expect the argument is that it's unnecessary. Technically, it is. But Zig's biggest weakness compared to other languages is that all the abstractions have to be in the programmer's head rather than encoded in the program. This greatly hampers people's ability to jump into a new codebase and help themselves. IMO this is all that's needed to remedy that without complicating everything.
You can see how much organizational power this has by looking at the docs for Go's standard library. Ignore how Go's runtime does all the work for you...think more about how it helps make the _intent_ behind the code clear.
Surely idiot investors are the easiest to get money from?