The point is the existence of that policy of positive discrimination makes it much harder for the female actor to prove herself.
The problem is that without positive discrimination, you don't get equality.
Don't get me wrong, discrimination is not good in and of itself. But it sometimes serves a useful purpose. In the past, there was explicit discrimination in favour of white male cisgender heterosexuals. People of colour, women, LGBT folks, were considered not suitable for one reason or another. Then the rules changed, and it became verboten to say it out loud, but there was still a lot of discrimination in favour of the aforementioned white men. Such things lead to the vast overrepresentation of white men in films, so the point where having a major movie which has either a woman with her own story or twonamed female characters talking about something other than a man is sadly infrequent (I'm not arguing that every movie needs to pass the Bechdel test, but a similar number should fail a reverse-Bechdel test aimed at male characters).
All of this leads to comparatively few women putting themselves forwards for positions of authority or prestige, because it is seen as something for men. We take racism, sexism and homophobia from our society, and it limits us. We see fewer males in "caring" professions because it's seen as unmanly. Actors refuse to play gay characters because they may end up typecast as such.
So we need to make discrimination work for us, rather than against us. Use it as a tool to ensure that there are voices from a variety of backgrounds in discussions, because that helps get a better end result. You use discrimination to shoehorn in people from differing backgrounds until such time as those differing backgrounds start appearing regularly in the non-mandatory positions. Once we achieve equality, there won't be any need for discrimination. But there's more to equality than just having the same rights - you need the same opportunities too. And right now we don't have that.
I wish that Whittaker's gender was not a story. It's a sad indictment of our society that it is, and I long for the day when such stories are unnecessary. But casting another white man is not the way to fix that.
Doctor Who is written as a TV series not adapted from a book for TV or the cinema.
