Sort of obvious why RSS went away. It is really hard to put ads into RSS. Sure you can just add a update but that's not really feasible (and could be filtered). So the large sites just started removing it and it helped them capture ad sales.
Therefore, RSS doesn't cause any advertisement loss, as the user needs to go to the website anyway (which, to be clear, is totally fine - RSS still works as filter/preview, which is very helpful).
Premium RSS is possible, with individual links per user, but it's much more complicated than just typing your email address into a field (something you already do when you pay for anything). Also, chances of the link being shared around is high and aggregators often inadvertently leak premium links. In contrast, someone's inbox is about as private as it gets.
In any case, as Netflix’s early history shows, depending on your cost structure, link sharing is likely a net positive. It’s probably only worth prosecuting the feeds that get shared with 100s of people.
It definitely is more complicated, but the tools to make it simple are easy to build! Maybe we’ll see growth in this space. One can dream
It's a good point that some amount of unauthorised sharing can be a positive. The main issue is it's hard to control without also having false positives, where the paying subscriber is annoyed by their service being revoked, having to re-auth, etc.
I can’t deal with most news sites now because I’m usually reading them on my phone. There’s just so much garbage between ads, scroll locked videos and JavaScript breaking the pages. AMP was supposed to help with this stuff but it breaks all the time too.
I know I’d happily pay for plain text content that I can format myself but I’m guessing there’s just no market for that sort of thing.