(look at how racists are reacting to the news of her casting)

I'm probably going to regret this but:

You are simplifying the various reasons someone could have issue with the casting decision by handwaving it away as "these people are racist", as opposed to understanding the reasons people criticise casting decisions. I will stress these criticisms can easily be hijacked by people promoting racist views under the guise of caring about other reasons

I feel the inclusion of minority characters abritarily is a PR attempt at "being diverse" instead of actually supporting diversity through other means. I don't know the best method to support diversity, but I feel this isn't the correct method, as it is highlighting there is an issue, while not actually taking meaningful steps to solve the issue.

Other reasons can include people.worrying they are not paying attention to the source material, people did overreact to this, but if they did cast Ciri as an asian woman (for example) [BOOK/GAME SPOILERS] then they would need to change other characters to be that ethnicity, which would give away the twist (if I remember the details of the story correctly)

Lastly some people felt that including minority characters to take a stand against the issue of racism is a lazy route and doesn't sufficiently explore the issue of racism in society, while still exemplifying problematic views on racism for people doing the barest minimum to say "we aren't racist", as you can use parallels and allegorical stories set in the world of the witcher through the discrimination and mistrust Geralt faces (and the monsters he fights) instead of having a black person in your show and acting like you are challenging the notion of racism.