Because XML puts the "type" of an object syntactically first, it supports this use case.
Secondly, when you need namespaces, they're a clean, simple solution to the core problem. When you don't need them, don't use them. Fortunately, unlike some of the DTD stuff which is hard to ignore correctly, it's really easy to not use namespaces. If the complexity comes from the problem, trying to solve it with a too-simple solution is not a virtue. You'll just reinvent the same thing, only worse, and with less library support.
For proof... see the endless poorly-supported attempts to do anything even remotely similar for JSON. The good news is that JSON is very simple. The bad news is that when it's too simple, you typically end up with a mess of poorly-designed stuff on top.
Given that you find three semantic rules to be "too complicated", I suspect you'll have a difficult time doing better.
[Edit: rephrase.]
-- Data model --
1. Every tag and attribute is optionally namespaced by a URI. (Tags usually are, attributes usually are not.)
2. The meaning of a non-namespaced attribute is defined by the tag to which it's applied.
3. The meaning of a non-namespaced tag is defined by the application.
-- Serialization --
4. To save space in the textual representation of XML, namespaces are (necessarily) referenced by user-defined aliases.
5. To save even more space, a default namespace may be (and usually is) defined for use by tags (but not attributes).
6. Alias definitions live in opening tags, and are scoped to the tag itself and all its children. They are of the form xmlns:{alias}="{namespace}", or xmlns="{namespace}" for the default namespace.
7. Tag and attribute names are prefixed with an alias to indicate that they live in that namespace: {alias}:{tag/attribute name}
8. If a tag name is not prefixed with an alias, it lives in the default namespace (if any).
9. If an attribute name is not prefixed with an alias, it is not namespaced.
10. The aliases generally have no semantic meaning, but there are wacky exceptions (e.g. XML Schema).
EDIT, forgot: 11. The "xml" namespace is reserved and predefined as 'http://www.w3.org/XML/1998/namespace'.