I think, actually, that Joe has fallen into the fallacy that XML is a suitable solution for a high performance project.
Simple observation ... maybe Joe just cares about performance of library software. As a library writer his decisions impacts a lot of other software, whether written with performance in mind or not. How can you not like that mentality insofar as it does not deteriorate correctness.
As for the further development of the XML syntax some people think it is already unnecessarily complicated. The W3C normally creates new syntax "around" XML, like with XML NS, XLink, etc. I'm looking more forward to broad EXI support; not that EXI looks simple but it has some desirable characteristics besides abolishing the human-readability aspect to XML; in the mean-time the "zipped XML" design pattern is mainstream.