I wouldn't be surprised if we get some control over monomorphization down the line, but if Go started with the monomorphization approach, it would be impossible to back out of it because it would cause performance regressions. Starting with the shape stenciling approach means that introducing monomorphization later can give you a performance improvement.
I'm not trying to predict whether we'll get monomorphization at some future point in Go, but I'm just saying that at least the door is open.