The myth obviously fails to take into account the enduring popularity of (for example) Black Beauty or Watership Down.
Talking animals (and their expressed opinions on human cruelty/stupidity) aside, I want both stories about people just like me (not being a member of those mythical target audiences), and about people who are different from me in one or many ways.
Often I realise the identity of at least one main character before, or soon after, I have the starting ideas for a plot and/or setting. Or I have an opening line in mind (whether or not I end up using it) that introduces at least one main character and at least one of their defining characteristics.
In deep space no one cares whether or not you can see. Stuart has a very strong sense of self. His blindness is a big part of who he is, but it doesn't make him inferior to anyone else.
Rob remembered the first time he kissed Zack... I had this idea of two boys, one black and from a privileged background, the other white and working class, and that they were going to have an adventure.
For Once An Assassin on the other hand, I knew a lot about the personalities and individual backgrounds of Lee Ann and the Captain before they had names, and before I knew anything about their racial backgrounds or how race was viewed in their universe as opposed to ours.
And in a yet to be written story, someone else gave me the age, sex, race and career aspirations of the two main characters (along with the 'together they fight crime' tag) long before I had any inkling of how I could build a plt around them.
As a very unscientific reader survey: in what way do you want characters to be the same as or different to you, and why?