In hindsight, it probably would have been better to define standard prefixes, let people just sort of register their own for non-standard ones in whatever manner is suitable for their particular top-level document type, and if somebody, somewhere out there did finally manage to stomp on each other, let that particular type of document where that happened deal with it.
While technically suboptimal compared to what currently exists, it would match people's expectations better, and in practice, I can't speak for everybody, but I just don't see a whole lot of documents with hundreds+ namespaces such that collisions are a realistic possibility. And when I do see documents with a lot of namespaces (XMPP, for instance, or XHTML+SVG+some other thing), there's still a top-level type that could keep its own registry just fine. A bit of guidance on naming extensions probably ("don't call it e:, work your name in somehow like with the initial of your company or something") would have 99.9% solved the problem.
Prior to seeing what happened I'd probably still have argued for the current namespaces spec. In principle it doesn't seem that complicated to me. But I'm obviously wrong in practice, because, like I said, I can hardly cite an example of them being used correctly at all.
(Likewise, in hindsight, entities shouldn't have been able to be recursive, and if we were spec'ing out the next generation of XML I'd straght-up remove them except for the ones necessary to XML itself, <, >, and & because UTF covers the major use case of entities now. I'd discard the "terrible, terrible templating language" use case entirely.)
