None of the good things you mention seem to require new investment in XML, I'm not sure what benefits you see in it.

Why not just maintain existing functionality in RSS, and at the same time design a new format than is modern, simple, and extensible? The modern format could be linked to or embedded within the legacy format as long as needed.

Newer clients could continue reading the legacy format while adding support for the modern format. Older clients would continue to work indefinitely.

This would allow a gentle transition and allow new investment in XML code to cease.

I'm not religiously against XML, but there are many practical benefits to not using it when possible.