Part of the disagreement here I think comes down to a confusion of author/book goals. Specifically, there are two options at play:
a) book/author does not want to tackle racism, etc. if it's not germane to the story
b) book/author does want to work against societal racism, without getting in the way of the story
Let's look at those two options separately, okay?
a: book/author does not want to tackle racism, etc. if it's not germane to the story
If you go with choice a, as a writer, that's fine. You don't have to make your fiction fight any kind of ideological fight. And it's certainly true that one thing an author wants is to keep this sort of issue from taking over his book, distracting from its actual subject. So I have a lot of sympathy for this position. But here's the problem. If you don't take on the issues (let's use race as an example), what does that do to your book? It's not actively racist to just avoid the subject in a single book. The book itself is fine -- it is not racist, in and of itself. You as an author are also fine. I'm certainly not going to call you or consider you racist.
BUT...there's an effect. Your book doesn't stand in glorious isolation. It swims in a big ocean of books. And sure, it's just one drop in that ocean, but cumulatively, all those drops are what make the ocean. And that's where the problem lies, because if you don't write a book that fights racist assumptions in your readers, then you have to accept that your book will join the legions of others that passively support the racist assumptions that are currently prevalent. Your readers will assume your heroes are white, unless you explicitly make them otherwise. Which is fine for one book on its own. But if 999 out of a 1000 heroes are white, in all the books you read, that is going to have its effect on most readers, especially on children. On a deep unconscious level, they will come to believe that hero = white. And they will envision themselves as the sidekick, or not on stage at all, and that will affect what they aspire to in life, what they work towards.
Example: Whoopi Goldberg tells the story of when she was a little girl, and Star Trek came on tv. She watched for a little while, and then went running into the kitchen, screaming, "Mommy, mommy -- there's a black woman on tv, and she's not a maid!" Original Trek offered the clumsiest kind of tokenism on this issue -- but it was an incredible first step, and had a tremendous effect. Sure, Star Trek didn't make a black person or a woman the captain of the ship -- that would have to wait for later incarnations of the show. But by just portraying a black woman as a competent bridge officer in her own right, it took a tremendous step forward in television culture -- and influenced Whoopi Goldberg and countless other little brown girls to believe they could do that too. Myself included. On a personal level, I'm tremendously grateful to Nichelle Nichols for taking that role, and to Gene Roddenberry for creating it for her. They were the first in the tv media landscape to show me what I could grow up to be.
As I said, this doesn't have to be your fight. Your book is just one drop in the ocean, and as one that implictly supports the overwhelming majority, its effect will likely be miniscule in contributing to the problem. Actively fighting racism, etc. -- those don't have to be your battles, and even if they are, you don't have to fight them in your fiction. It's okay.
But what about those who do fall into this group:
b: book/author does want to work against societal racism, without getting in the way of the story
This is where it gets really hard as a writer. Because if you care about fighting racism (and all those other -isms), your fiction may well be the most effective chance for you, as a person, creating some small good change in the world. But as an artist, you have to balance that with not compromising your story or your characters, with not simply turning your fiction into a soapbox -- because then, frankly, you might as well just write nonfiction and be done with it.
Speaking only for myself, I do think about these issues with every story I write, and then I try to do what I can, without compromising the story. For me, the art comes first. But that said -- I still find that I can do a tremendous amount, if I work hard at it, and am perhaps just a little clever.
So John -- it seems clear to me that you, as an author, fall into group b -- you would like to fight the good fight in your fiction. You've been trying to fight it already. So then the question is simple, though the answer is complex and delicate to navigate: how can you best tackle these issues without compromising your story? I think you can be more effective than you have been, if you take a different approach. And that's what Kate and Tempest and the others have been trying to argue.
So, have they succeeded in convincing you?