1. Everything was perfect on paper is probably one of the reasons it tanked. 2. How the fuck is Johnny Depp playing a native American perfect on paper. Some people.
Hugely popular actors sometimes pull it off without much backlash if done tastefully. Did you notice that Robert Downey Jr. acted out most of Tropic Thunder in black face? I mean... that's pretty much the gold standard of all kinds of wrong but it didn't kick up much of any kind of dust because of the way it was done (in this case, it was meant to be a farce and the movie spent time jabbing at him).
I assumed it was going to be one of those "raised-by-Native-Americans" scenarios. You know, a reboot prerogative that would have been easy to make. Or at least that he would be heavily directed/scripted to keep things tasteful. So on paper, it was potentially a positive and don't think that people didn't go see this movie because Depp plays a Native American. He's the only reason many people are watching it at all. Had the movie been really good, people would have gone in droves.
Note, however, that this film has a HUGE disparity between critic reviews and customer reviews:
Assuming that companies haven't figured out a way to throw the numbers off, a 27% critic score to a 64% audience score is significant (237% higher than the critic score). Not only that, but bad reviews usually skew audience scores downward because a lot of people can like the movie but be persuaded after/before the fact that it isn't good by reviewers they trust. So the audience enjoyment may have been higher in all honesty. But to put that in contrast, Grown Ups 2 got a critic score of 7% and an audience score of 90%. I think Lone Ranger's sin is being ok and not fantastic. You don't spend hundreds of millions on something and produce mediocrity. Even the Last Airbender in all of its awfulness was bad enough to generate more views. No one talks about boring.
But yes, everything being perfect on paper is likely what led to lazy writing and directing. Shame, I like westerns.