I hate to break it to you all, but people who don't want to see an earth-first storyline are the minority. Most people tend to enjoy stories that have some sort of real-life stakes and moments we can relate to. Sure, you can relate to Thane as he's dying or Mordin as he tries to attone, but ultimately, if you want a HUMAN audience buying and playing your game to really truly relate to your story, the vast majority of people want it to be a human-centric story.