If a movie is going to flop because a particular actor isn't in it, then I think you have a much bigger problem with the movie itself rather than the casting. If the only reason to put people in a seat is by who is in the movie, then you clearly have a problem with the concept and script of the movie.
As for Halle Berry and Storm, at least she is clearly not white, though of mixed race. I didn't care for her cast as Storm, nor her acting in the movie, nor the script for her, but at least she wasn't a Nordic looking person playing a black woman. (And Storm is full African, she was found there, born there, lived there, and I doubt she ever got her Visa unless Charles implanted a few suggestions in some border agents)
Oh. I did a quick Google and it said Storm's Mom married an American journalist and had Storm in Manhattan. >_> *shrug*
I agree with the faults of the movie if they need to rely on casting, but before anyone decides to see the movie there are too many factors on the surface. The overall concept and script aren't what get people into seats. They are the biggest parts of what make a good movie, but you can't judge either of those by a trailer or promotional images. What people usually consider in advance...
Do I like this style of movie or its story?
Westerns aren't too hot nowadays.
Am I going to like these characters?
Usually if you don't like the actors, there's a high chance that the answer to this is no.
Do I want to see these actors?
Hardly anyone knows who Armie Hammer is, a random albeit good NA actor for Tonto would be in the same category. And then if they hired a superstar for the Lone Ranger and casted an unknown as Tonto, I'm sure there would be debate on that too, with those considering the casting of his role as minor and unimportant leaving him to be portrayed as 'that Native sidekick', by someone that the masses don't care to see.
I guess I just keep repeating myself. Out of the stars that draw an audience, I think Depp is probably the most suited for the role and they needed something to make this movie marketable to the masses.
In the end, I think having Depp as Tonto will not be a decision they'll regret. I'm presuming that the movie will be moderately entertaining ( I don't like westerns ) and that a lot of its gross is going to be from older Lone Ranger fans or current Depp fans. Take Depp out of that equation and your biggest draw is older Lone Ranger fans and fans of westerns...I can't speak on the size of the Lone Ranger fanbase but from the most recent box office results, westerns don't draw much ( HAHA no pun intended ) unless it has talking animals in it... and even Rango didn't do that well. In fact, I'd say they're almost a gamble to begin with. Rango did 245 ww, Cowboys and Aliens did 100 and Jesse James did 15.
The only downside to the decision of casting him is the handful of people complaining 'but he's not a real Native American'. If that's enough to turn someone away from the movie if they had been a fan in the past, then I guess they weren't much of a fan and that's one less person from the smaller demographic that they're relying on audience from, but overall I don't think the negative thoughts of those people will be enough to hinder what mediocre success ( I REALLY doubt westerns, can you tell? ) the movie may have.