Well, assuming that some indie devs make nothing at all, and some indie devs are Notch, I'd say, anywhere from zero to $1.7 billion.
Look at the "real" indie games market (for the sake of argument, exclude anything not sold on Steam or GOG), and things look much more favorable.
When you remove all the student games, the angry birds clones, the board-game clones, and the just plain terrible games, you're left with a much smaller pool of competitors.
Being an indie developer is like being an actor. Everyone thinks that it's all about luck, but they forget that 99 out of a 100 people who move to California to make it in Hollywood, can't act their way out of a paper bag.
If you move to LA, and you have actual acting talent and the looks to match, chances are you can make money--maybe you won't be a huge star, but you've got a good chance at earning a living.