RSS has always been a niche solution used only by technically knowledgeable people because of how it was always presented by RSS feed vendors.
Instead of having a button saying "Install this browser extension and click here to get all new articles in your browser", the user is presented with an orange "RSS" button with no explanation of what it is and how to use it, that will show a weird XML file when clicked on by the average user.
How are people supposed to use that feature when it's so obscure?
Better UX would have helped adoption and would have led to Google keeping the RSS button (and Google Reader probably).