I can't imagine why. XML is still fundamentally, well, a markup language, not a serialization format designed as such. But the "extensible" part isn't so accurate - attributes aren't extensible. GP complains that JSON doesn't know what an integer is (as distinct from a generic number), but at least it does know more than just strings. And needing to repeat a tag name when closing it just adds useless complexity.

It’s not anymore useless than a closing } or ], except since it has the tag name in it, so when I’m reading a highly nested object I’m not stuck in my text editor looking at a bunch of }’s at random indentation levels I have to scroll all the way back up to regain any context for. Tags are text which is visual structure I can choose to read, or choose to gloss over and use as bulk to shape the data in my head.