Why can't you put ads on RSS? Either in the story itself (by the site or aggregator) or as a "promoted item" in the feed (by the aggregator). If anything, the Google Discover (or whatever it's called) is not too different, just that you don't control which exact news sources you're subscribed to.

I can imagine an alternate timeline where Google Reader turned into a sort of Twitter (or FB or IG) feed.

I'm just here to say that I'm still bitter about Google Reader. :-(