I’m a heavy RSS consumer, but I’m absolutely in a minority. RSS features according to Mozilla’s own telemetry are used by an extreme minority of users (I can’t find the exact figures but certainly less than 0.5% of people), and are a fairly large and unmaintained part of a codebase that’s undergoing extensive modification. I’d far rather Mozilla expose some APIs and let people make RSS-based extensions than have it built in and getting in the way of performance work.