Many positions people take have a "good" reason and a "real" reason. The good reason is the one that seems less selfish, like "you can't understand my experience." People may disagree about whether the good reason really is a good reason, but the argument is at least substantive.

The real reason is more selfish; it's why the matter being argued about matters enough to the complainer to be worth complaining about. That's the "preserve" argument. I am suggesting that Black writers in particular want to carve out writing about Black characters as their turf so that they can actually earn a living writing. They offer the good reason - which you and I, I think, don't find terribly persuasive - because they can't practically offer the real reason. But I believe we owe it to people to suss out their real reasons and then consider whether we wish to make concessions to its purpose, in this case, by letting Black writers write Black protagonists.

You may decide not to leave writing "others" to those others. As you say, you're against segregation. ("It's every man for himself!" cried the elephant as he danced among the chickens.) I am against telling people what they cannot write about, but I do have sympathy for writers whose identities make commercial success difficult. OTOH, I'm not a writer, so I don't have a dog in this fight either way.