I was just looking up River Phoenix's biography entry online, and it said that if he hadn't died he would have potentially played Jim Carroll in "The Basketball Diaries", Arthur Rimbaud in "Total Eclipse" and Jack Dawson in "Titanic". All these roles, of course, later went to Leonardo DiCaprio due to Phoenix's death.
If Phoenix hadn't died and had taken these roles, where do you think both he and DiCaprio would be? Would Phoenix be bigger than DiCaprio is now or at the same level? Would DiCaprio be as big as he is now or would he be the Chris O'Donnell to River's Leo? Who would be the more famous of the two - Phoenix or DiCaprio?
What happened to River was utterly tragic. He is imo one of the best actors off all time and had he lived would I have no doubt carried on giving remarkable performances.
But Leo Dicaprio is a good enough actor that even with out those roles you mention he would likely have worked his way upto the level he is today. By the time Titanic had been released most of my school friends were already in love with him thanks to Romeo and Juliet.
There are many actors (Brad Pitt, Johnny Depp to name two) who would have lost roles to River had he lived, not just Leo. He really was the leading light of that generation.