Once you learn about Venn diagrams it's going to completely blow your mind. One easy first pass at making inferences as to best services is to use popularity as an imperfect first proxy. So if someone says that a service became a de facto standard used by all of the internet, it's one of the ways to helpfully frame the conversation about whether the service proved to be useful. Of course there will be exceptions, but it's one of those functional literacy things where everyone can understand the significance of why people would bring that up in the context of a conversation that ultimately was closing in on making assessments about the value of the user experience.
Google's RSS reader was simple, ad free, fast, had an elegant design, didn't attempt to push superfluous services or subscriptions, set a standard for accessible anywhere at a time when many popular RSS readers were used on the desktop, and, in contrast to most of the best services now around, was free. I would go so far as to say it was almost unimproveable because it had one job and it did it correctly and didn't try to do other things, which used to be one of the things Google did best.
And I feel like none of this information is new, hard to discover, controversial, but is instead part of the generally accepted canon of internet history. So it's a bizarre question to me, if I'm being honest.
Edit: regarding the prevalence of RSS adoption in the present day, it may be true that as a numeric total there are more sites using it, but that as a percentage it is down. The same way that a winning candidate for president from the 1940s will have fewer votes then a losing candidate from the present day. Numbers will grow over time just due to population growth. Or in the case of the internet, The growth in the number of sites. But to understand whether RSS enjoys the same status as a de facto standard it's necessary to look at more than just the numbers, but at the proportion of adoption today compared to the proportion as an existed in the past.
