The example may be unnecessarily verbose (for example, they would have saved 90 characters by using json as the default namespace), but this is text. It's ridiculously compressible. Saving a handful of characters here or there is probably wasted effort.
There is interesting side to this example of yours. Terseness and its power have never been appreciated by the masses. The average programmer needs a tool, that can match his level of competence. This isn't just true in Data formats but also true when it comes to technologies.
Java and Python, are loved by the masses today for that very reason. So as long as eclipse and its Ctrl-Space takes care of 'filling up the blanks' most programmers will use it. Beyond that everything is irrelevant at least for the masses.
Now one fine day a cool Perl/Lisp/C programmer comes around and shows a you how you could do all that with more compact code along with flexibility and power and people suddenly realize what fun they were missing for years.
Verbose stuff tends to get very famous for a while and then just fades away with time, never to be seen again.
muk:~ fiam$ echo '{ "name":"John Smith"... }'|wc -c
303
muk:~ fiam$ echo '<?xml version="1.0" encoding="UTF-8"?> <json:object> … </json:object>' |wc -c
904
I don't really have anything else to say.
Edit: formatting