XML has worked just fine for RSS. The reason why RSS isn't as mainstream as it used to be around 2004 has nothing to do with XML, but is a consequence of Google

XML has worked just fine for RSS. The reason why RSS isn't as mainstream as it used to be around 2004 has nothing to do with XML, but is a consequence of Google capturing, than abandoning the audience, and of walled-garden social media taking off. Inventing a new JSON-based format in this niche is as helpful for adoption as matrix.org is for federated chat - it will only serve to fragment the space.