The reason for [copying literal result elements with all their
in-scope namespaces] being the default behaviour is that the processor
in general can't tell which namespaces are used and which aren't (it's
not just prefixes-in-content; there have actually been some XML
vocabulary designs in which the mere presence of a namespace
declaration is used to carry information).

Now that's even more subtle than putting the xmlns attribute (and thus
the namespace declaration) in the DTD!

Of course, you could combine the two subtleties by declaring a namespace
unused in markup or content by unobtrusively putting it in the DTD:

Whatever the merits of this approach, thanks for the useful additional
information.