I wrote about this recently[1]. The "problem" with RSS, that essentially lead to it falling out of favour, is that it is pure consumption, not interaction. You can't "like" an update that comes to you via RSS, nor comment, nor easily share. It's difficult for sites to monetise it, or track, or promote their own agenda. It's not "social" and since every other site these days needs to be social, whether through aforeme…
And this is why Google Reader was amazing. There we were, my friends and I, reading RSS feeds and liking/sharing/commenting amongst ourselves. pours one out
Note: fairy new to Mastodon, not sure exactly if / how this would work, but I've been thinking about this lately.