If we forbid now, we have that syntax available in the future once we know what we want to do with it. By the way, forbid is not keeping things constant. It seems to add complexity with no clearly defined use case. The few examples of documents that user relative URI references in namespace declarations do not move me very much. John Cowan believes as a matter of principle that even one document is justification for keeping the old syntax legal, but I'm not convinced.
Incompatible changes are sometimes the least-disturbing way forward.
I'd expect almost everyone in the world to immediately jump to namespaces 2.0, given the practical impact, as seen from the discussion so far, so in practice, I don't think it makes any difference. It's just a way to reclaim the syntax for thoughtful, planned re-use rather than hastily resolving the problem in a way that we may well regret. We need not recapitulate the whole history of the namespace recommendation in the debate of this single issue, unless we want to.