Do we really need this? Atom is fine for feeds. Avoiding XML just for the sake of avoiding XML, because it isn't "cool" anymore is just dump groupthink.
If this industry has a problem, it's FDD - Fad Driven Development and IIICIS (If It Isn't Cool, It Sucks) thinking.
Part of me is with you. But even in established languages I've had trouble finding an appropriate xml parser and had to tweak them way more than I thought necessary. I haven't (yet) had that problem with JSON.
I think with something like feeds there's the possible benefit of becoming a 'hello world' for frameworks. Many frameworks have you write a simple blogging engine or twitter copycat. I don't think I've ever seen that for a feed reader/publisher. People have said that Twitter clients were an interesting playground for new UI concepts and paradigms because the basics were so simple (back when their API keys were less restrictive). Maybe this could be that?
But even in established languages I've had trouble finding an appropriate xml parser and had to tweak them way more than I thought necessary. I haven't (yet) had that problem with JSON.
Maybe it's just that I work mostly with JVM languages (Java, Groovy, etc.) but I haven't had any problems with handling XML - including Atom - in years. But I admit that other platforms might not have the same degree of support.
The Hitchhiker's Guide to Python[1] (a popular reference for Python) recommends untangle[2] and xmltodict[3], neither of which I've used.
I feel like in other languages I've used had similar brittleness when dealing with xml. I might be biased because working with xml in an editor it's difficult to validate visually or grok in general when used in practice.
[1] http://python-guide-pt-br.readthedocs.io/en/latest/scenarios...