Despite some warts, XML and the Web have done pretty well. They work and they work well. A large part of that is because both were designed with certain basic principles in mind. This gives them a unifying vision and a clean architecture that solves many problems.
However, when a technology becomes successful it often attracts developers who recognize its success but don’t recognize or understand the underlying reasons for its success. Each one wants to make a change here, an addition there, a deletion somewhere else. Sometimes these suggestions are good and valid. Sometimes they’re not. However, even the suggestions that address real needs and use cases cause problems if they’re made without a deep understanding of the principles of the thing being changed. It’s like modifying a building by knocking down walls, cutting new windows, and erecting an extra bedroom on the roof. If you do this without consulting the original blueprints and understanding of the architectural principles that went into the house design, the best you can hope for is an ugly mess. More likely the whole structure will collapse around you, as the changes weaken the foundation the whole edifice rests upon.
Previous examples include cookies, frames, SOAP, YAML, SimpleXML, binary XML, RSS, and many other cases I could mention. However the latest is coming from a place I really didn’t expect it: the W3C XForms and XHTML working groups. These two are working together to eviscerate XML namespaces, and make it difficult to impossible to process XHTML2 and XForms with standard XML tools like XSLT and DOM.