I keep coming back to the “You don’t have to be real to be the Doctor” line from Extremis, because it is just so damn good, especially if you take it out of the context of the episode, and look at the casting of Pearl Mackie, and Bill being made explicitly gay. I think if it had been in any other season, it wouldn’t have been nearly as poignant. Yes, it still would’ve been sort of an uplifting “the Doctor doesn’t have to be a real person to do good” type of message. Which is nice, and true: there’s lots of stories of people for who the show is a source of inspiration and happiness.
But take into account the recent push for better representation on the show, and then think about all the people who say that it’s SJW nonsense, that it’s pandering, that it doesn’t matter who the main character is because they’re fictional. They don’t affect people, because they’re not real.
Except we know that’s bullshit, of course. When Pearl Mackie was cast people were really happy, and when Bill was revealed to be gay, people (including me) were ecstatic. I’ve seen lots of people who said they were watching the show again just for her. There are so many people who identify with her, for who seeing someone like themselves as a main character is so important.
Bill is a fictional character, yes, but she doesn’t have to be real to matter.