If popular browsers had embraced RSS enthusiastically, it might have taken off with mainstream users. There's nothing about the protocol that makes it difficult to provide a well-integrated experience. Firefox even had a crude built-in reader around the time it was the most popular browser, but did not put in the effort to create a good UX around it.
People are definitely used to poking cryptic pictograms on contemporary UIs, and RSS clearly gives off an impression of something being broadcast.
https://en.wikipedia.org/wiki/RSS#/media/File%3AFeed-icon.sv...
The UI and UX was left entirely to the website administrator, not big tech, and even websites that did not have any monetization did not improve the experience for users.
And of course adtech could have invested in RSS to make it financially attractive. But they didn't, cause nobody uses it.
The most obvious people to have improved that UX is the browser... aka Google. The way that XML rendered was controlled by the browser. This all sounds like Google apologism.
Firefox had Live Bookmarks, which I used for a long time. You'd just drag the icon to your bookmarks toolbar and then it would appear as a folder containing all of the entries as clickable bookmarks to the relevant web pages. The browser alerted you to autodiscovered feeds as well. The orange RSS pictogram (not the initialization) would appear right there in the URL bar if the site was set up right.
As early as OS X 10.4, desktop Safari had a built in RSS reader as well. You'd open the sidebar that's currently mostly used for the reading list and bookmarks, and there was some way to add the current page's discovered RSS feed with a button click or two. It also rendered feed XML in a particularly nice way that looked like a very clean looking blog, so landing on an XML page wouldn't intimidate less technical users.
Chrome deliberately was dysfunctional, and it taking over probably had more to do with RSS not growing more mainstream (as well as the rise of social networks and over commercialization) than Google Reader shutting down.
And even if the feed was rendered properly, it's essentially useless without an associated extension or website to aggregate those feeds.
Oh, and Opera mini had a built-in RSS client too.
It's not surprising. It seems that many understand RSS more as a form of news (like a newsgroup or mailing list) than as a form of discovery for web content.
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).