This being the very last song that Pink Floyd has written to date (and its placement at the end of their last album of new material to date) lends itself to the interpretation that the song narrates the story of the band's career, from their beginnings to their grandiose success (with The Dark Side of the Moon and The Wall) to their breakup to their carrying on. The final line ("The endless river, forever and ever") ties into one of the band's first hits, "See Emily Play" ("Float on a river, forever and ever"). Wikipedia.
To borrow a comment from YouTube, "Shine on Richard, you'll be missed."