DiCaprio could've played the Tom Hardy role. Or, if we're further fictionalizing the story, a made-up role.
Marketers don't necessarily care whether a star plays a leading role or a supporting role. E.g., Johnny Depp in Into the Woods.
If DiCaprio's in it, they can sell it as if he's the star. Because audiences don't care that much either.
DiCaprio vs. Depp
Someone else chimed in:
"The sad truth is, Leo [Johnny] sells tickets. He gets movies made. You couldn't realistically make this flick without him. With a budget of about 130m, you need to make sure you're going to get a certain amount back by casting a face who puts people's butts in seats."
The "selling tickets" part isn't a sad truth, it's a wish or a hope. An article of faith. The truth is that except for maybe Tom Cruise, no white actor can guarantee a hit movie. Certainly not Johnny Depp, whom Hollywood considers a hitmaker. These actors can't put "butts in the seats" and "make sure" a movie earns a profit.
I checked and DiCaprio's movies have a solid record of earning profits. But he generally does prestige projects with the likes of Martin Scorsese, Christopher Nolan, Steven Spielberg, and James Cameron. I.e., big names who often have hits because of their directing and producing skills regardless of whom they cast. DiCaprio doesn't take many risks with low-brow popcorn fare like The Lone Ranger.
So The Revenant was a gamble, not a sure thing. A $60 million film with a Native star, DiCaprio as the co-star, and a big-name director also would've been a gamble. We won't know which is the bigger gamble until DiCaprio and Hollywood actually make the second movie.
The "gets movies made" part is the sad truth. Because Hollywood ignores the success of diverse films and continues to operate via its old-boy network aka racism.
In short, the facts are these:
Diverse movies are a huge business. Why doesn’t Hollywood make more?
Not whatever Hollywood thinks they are.
For more on Leonardo DiCaprio, see The Revenant = "White Savior" Story and Is The Revenant a Game-Changer?