The core feature of a browser is browsing. Web dev tools are secondary. To a regular user who is not dev the removal of RSS is a loss. JSON is secondary. Firefox would make a mistake if they prioritized Dev tools over end user features.
How many regular users actually use RSS? To be honest, I don't think most devs use RSS, much less non-devs.
Mozilla has a finite budget to allocate to features. Maybe they looked at usage rates and decided that RSS just wasn't an important feature, or other features were more important.
