The point of RSS is automation. RSS is an established standard1 so that no human needs to keep revisiting a long list of vetted topical sources that regularly publish worthwhile content. With RSS, a computer can rapidly and flawlessly determine whether another computer offers new content. If available, the content itself can be automatically retrieved, too.
Human link sharing is different. When humans share links, those links are likely intermixed with irrelevant items like unsolicited opinions. The links may be about a wide range of uninteresting topics. Although link sharing distributes the work of revisiting valuable websites and flagging fresh information, the flaws of a manual procedure continue to apply. How reliable is the crowd at performing this task? As a unit, they're more reliable than one person, but I daresay that a program is better still.
RSS is for mechanizing the syndication of "publishing sources". I'd say that humans who share links are more accurately classified as additional publishing sources than as "John Henry competitors" to efficient RSS software. I'll concede that link sharing can be a more convenient answer than RSS to the question, "How do I obtain a simple list of interesting links to visit?" But RSS is superior if the question is, "How do I, a unique individual, follow the ongoing activity of every source in a list that I control?"
Shared links won't replace RSS for my needs. To suggest otherwise is to misunderstand the purpose of RSS. When a website shuts down its feeds due to "lack of interest", I'll gladly turn my attention elsewhere.
1Ha! RSS is a standard in the sense that you pick whichever one you like.