I think, given that Arc is supposed to be "a language for good programmers" , that it's silly to impose restrictions like withholding a namespacing mechanism to encourage a certain type of programming.
Though I like keeping codebases minimal, I also like freedom.
To my mind it is namespaces that add restrictions. Arc's warning about redefining names can be ignored, but a namespace mechanism is usually more insistent about violations.
I try not to use words like 'freedom' in these discussions because navigating by them is a subtle business. While having a feature can sometimes provide freedom, in general I tend to assume that features cost freedom. Like the saying that your possessions own you. A namespace mechanism is more code to write, more places for bugs to be hiding in, more errors for the user to run into, more places for one programmer to hide things where they can trip another programmer up. All these issues are places for degrees of freedom to be used up rather than created.
So yes, we are absolutely in agreement that Arc is for good/experienced programmers. But there are multiple visions out there for how you go about helping experienced programmers. My vision is admittedly unconventional, a minority view. It's possible that Arc just doesn't have namespaces because the authors haven't gotten around to implementing them yet. But somehow I doubt that.