"io.Reader and Writer are interface{}'s[1]."
No, they are interfaces, but they are not interface{}s, the interface that is met by having no methods and is therefore met by everything. Nobody is complaining about the use of interfaces that declare a useful set of methods, and are meant to be used when the set of methods is all the target code cares about. They may complain about some of the edges around them (no support for contra- or covariance), but that's not the complaint that is being discussed here.
The complaint is specifically around interface{}, which is a type that means nothing, and so everything fits into it. It means that if you are a function that is receiving an interface{}, you know nothing about the argument to start with, and will have to either pass it along to something else, or examine it via type assertions or in the worst case, the reflect library.
As a multi-year Go programmer, I agree that A: the "true" interface{} that is the meaningless type appears less often than people think, and that if you program is shot through with it, you are either working in a domain where Go is not suitable (complicated math processing, IMHO, for instance), or you are programming some other language in Go, and need to learn how to use Go properly, but also, B: it does happen that things that would be well-typed in other languages will have interface{} show up, which means you're going to sacrifice runtime safety and some performance, though the exact impact depends on what you are doing.
(I say the "true" interface{} because there's also some places where interface{} shows up, but it doesn't hurt you. For instance, the standard JSON library accepts an interface{} to say what data structure to parse the JSON into, and the code that implements that is somewhat complicated, but for the most part, that interface{} doesn't bother you, the Go user. In theory you can encounter run time errors if you pass in something that doesn't work, but in practice, if you pass in the same type every time, which is the natural case that is going to result if you just program normally, it's not a problem. In general, `interface{}` showing up in the incoming parameters of a library is much less concerning than an interface{} showing up in the return parameters for a library. In the former case, even though the language does not enforce type safety, you can enforce type safety by how you use the method; in the latter, you are stuck with an interface{} in your code with all that implies.)