Both good points! Upon reflection, I guess my main thought here is that from the publisher's perspective, it's difficult to provide a "subscribe with RSS" button and have it be as frictionless as a typical email subscribe form. e.g. even if the user does have an RSS reader, the publisher has no way to know that, let alone which RSS reader they use.

I was thinking about Mastodon while I wrote this--IMO it would be a more approachable experience for many people if Mastodon profiles had a typical email subscribe form where you could put in your own Mastodon username + server, in the same format as an email address (i.e. without the leading @), and then have it send a request to your server. Then next time I open Mastodon I'd have a notification with an "approve" button or something.

Of course that may just open up a whole other can of worms around spam signups, so that may or may not be a worthwhile train of thought.