About the namespace problem: I do a lot of refactoring, and namespaces were never my problem, because the types of errors op is talking about could be handled by the test suite, and/or cider reloading. I really think I would not want versioning of my namespaces - that would make a huge mess. So like many other ideas that sound good and pure and elegant at the first glance, I think this one would be an overly complex mess in practice.

BUT, there is a more important thing, and there I support the author: instead on pushing Rich (the author of Clojure) to include any shiny feature that someone thinks would be nice, and then complaining that Rich ignores the request, the OP forked Clojure and tries to prove that his ideas are better. I think Rich does quite well on keeping Clojure sane and useful. I have no doubt that he will include stuff that really proves to be better. But, 99% of stuff is better only while is on scratchpad, but turns out either irrelevant, or, even worse, outright horrible in practice.

That's why I think that everything that can be a library should be a library instead a part of clojure.core. Those things that could not be libraries (not many of those come to my mind) should first be proven in some sort of "experimental" clojure. clojure.core should stay as it is :)