There's nothing wrong with very little plot in theory. Over the years there have been plenty of movies that manage to pull off something special with a story that can be summed up in a sentence or two. Richard Linklater's Dazed & Confused, for example, is about a group of teenagers hanging out on the first day of summer vacation. Sofia Coppola's Lost In Translation is about an ageing movie star who meets a young college graduate in Tokyo and repeatedly hangs out with her at a bar. In this way, The Revenant is about an injured frontiersman who survives a vicious bear attack and attempts to make his way back to the central trading post Fort Kiowa. The problem is, it just isn't enough... especially not for a film that's two and a half hours long. Looking at the source material, it's easy to understand why this is the case. The movie is based in part on American author Michael Punke's The Revenant, which in turn is based on the true story of Hugh Glass. In the book (and in reality) Glass doesn't have a son, nor does he find Fitzgerald when he makes it back to the Fort. The film adds the son as motivation and throws in the final pursuit of Fitzgerald for narrative satisfaction, but the middle of the film is largely devoid of any actual story. This is particularly egregious when viewers are teased with an interesting back story involving Glass's wife and his time living with the Native Americans, but it's left frustratingly ambiguous.
