This is the core issue with XML/XMPP. UX simply doesn't seem to have ever been a factor when designing either of them - whether towards the developers or the end users.
(XMPP would've used JSON had it existed in 1998, fwiw.)
Yes – in particular, I remember liking the initial promise of XML 1.0 and then being disappointed as the initial simplicity was buried under layers of gratuitous complexity as the enterprise computing crowd jumped on board. Some neat ideas (e.g. XPath)
Similarly, my first database-backed web app was a Java servlet in 1996. I assumed it would become richer in the future but it was still pretty easy write and deployment was pretty simple at that point. Instead, the community chased off after more complexity (JavaBeans, the near-pervasive obsession with runtime configuration and the ensuing massive config files, nested abstraction layers, etc.) rather than focusing on basic developer productivity. We got massive piles of code like RMI (i.e. CORBA) before basic features like regular expressions, logging, etc.
> Could we have built anything on the scale of the web with CORBA?
No. We also failed, repeatedly, to do it with XML for reasons which are obvious to anyone who's ever used SOAP/WSDL/etc.
The common theme was seeing complexity and, especially, abstraction as a universal good rather than something with real costs and forgetting how important high-quality implementations are, particularly when you're relying on them to help with all of that complexity.
Rich Hickey did a great service outlining some common problems when thinking about the word complexity itself: http://www.infoq.com/presentations/Simple-Made-Easy
I don't think it's about the engineers wanting to see complexity, so much as the problems you mention stemming from design-by-committee.
The web is a lot of things, but broad consumer adoption of web technology also involves boring things like banks. SOAP and WSDL weren't created in a vacuum. They exist because Java / XML made it even possible to bridge the gap from the old world.
No, it's not the biggest challenge but it's so transparently a case of conplete disregard for developer productivity that I've heard it mentioned by a lot of people as why they avoid XML.