I have already justified my preference for jsonfeed. XML has proved many times to decrease developer productivity. To my knowledge open source projects that have dropped or diminished their use of XML have only become more popular over time. Extrapolating a little bit, switching to JSON can only be a good thing.
If you're such a hacker why not just write a script for yourself that convert javascript to xml instead of making a new standard and fracturing things even more :P
Or would you care to share a hack you've done with jsonfeed that I can't do with my RSS reader?
Historically, XML is infinitely better than the custom formats we had before, where might even have been in binary instead of textual (the horror). Of course they went overboard with XML (oh hello XSLT).
But RSS feeds are a prime example for a good use of XML. Standard, validated, machine-parsable, and easily extendable data structures. For example podcast feeds are just extended RSS feeds.
I don't think that projects got popular because they dropped XML but because they dropped unnecessary baggage.