Well, I think that we have to keep in mind that the main reason we've never had a female (or black/Indian/hispanic/whatever) Doctor before has nothing to do with Time Lord gender identity. It's that until pretty recently, it was inconceivable that anyone but a white male could be cast in such a role, and even now, it's obviously still controversial. So in a situation like that, do the writers A) say "But we know better now, and we can and should consider a wider range of actors for the part, even though it will be a break in tradition," or B) say, "Well, regardless of how crappy the reasons for it being this way are, it IS this way, so we have to keep it this way?"

Basically, when the only reason people can advance for keeping something like this the way it always has been boils down to "It's always been like this, so changing it would be weird and scary!" I'm dubious. Change IS weird and scary, but that's not a reason to avoid it, and may be a reason to embrace it.