From a user perspective, the thing about RSS is that most readers/clients were designed as if they were email clients, where you mark articles as read/unread. Who wants thousands of extra "emails" in their inbox to check every day ?
From a site perspective, most sites were stuffing the whole web page in each RSS article such that RSS was not a summary. Then they realised that people reading RSS weren't reading ads so they either killed RSS or made each article one line.
From a programmer perspective, writing an RSS client became a hello world of applications programming such that there were millions of very bad clients.
RSS is very much alive and very useful; but maybe RSS as we used to think of it is dead. It's a background thing that browsers and applications should make use of
I think the big challenge for RSS and the web in general is link rot.