I'm not sure how significant the nominal vs structural distinction even is. In Go, a struct can implement an interface without declaring it, but the programmer still needs to deliberately write the struct to conform to the definition of the interface, so they're still coupled, just not explicitly [1]. Yes, it is possible to define a new interface which fits existing structs which weren't designed for it - but how common is that? That is, how common is it for two or more structs to have a meaningful overlapping set of methods without being designed to conform to some pre-existing interface?

[1] which is obviously a bad thing

> That notion quickly grew into the idea that value types had behaviors, defined as methods, and that sets of methods could provide interfaces that functions could operate on. Go's interfaces arose pretty much right away.

If you replace Go with Java, this would've accurately described Java ~30 years ago.