First of all, in-story, it seems like that sort of regeneration would require more control than the Doctor’s ever had. All of his regenerations have been white males appearing between 22 and 55 originally. (Hartnell was 55, and as the original his age could be partially attributed as such). In River’s three regenerations, we see her go from seven to “a toddler,” and from early twenties to forty or so, including a change of ethnicity in the former. That might be due to her human-plus nature, but it also shows a wider range of regeneration change than the Doctor has ever done. I think if he wanted to change (even ethnicity, but especially gender) he’d have to specifically will it, otherwise his “template” will come up with a new model relatively like the old.
Outside the story, I think it shows a misunderstanding of the show and disrespect to other female characters. Would we want Star Trek with Captain Jane Kirk? Should we have Iron Woman or Nicole Fury? What about Lucy Skywaller and her friend Hannah Solo? No, we value those characters as characters, and changing them would be only a gimmick. Likewise, I wouldn’t want Death from Sandman to suddenly be a guy or have Lucy Pevensie replaced by Luke.
Furthermore, it actually de-values the other strong female characters the show has had now and in the past. While many people say the classic companions were “screaming, frightened women,” that’s not the truth. Sure they screamed—but so did the guys, including the Doctor. And they did some pretty impressive stuff.
- Zoe talked a computer to death
- Amy remembered the whole universe back into existence, including the Doctor
- Barbara ran over Daleks with a truck
- Donna knocks out a Sontaran with a hammer
- Ace attacked a Dalek with a cricket bat
- Leela carried a pouch of poisonous thorns with her and ended up marrying a Gallifreyan (for reference, the last two companions the Doctor brought to Gallifrey had their memories erased.)
- Nyssa held the Lord President of Gallifrey at gunpoint
- Jo Grant resisted the Master’s mind control by reciting nursery rhymes
And that’s just some of the companions.
If the Doctor has to be a women to prove that women can be strong/leaders/brave/etc you’re implying that only the Doctor demonstrates those qualities, instead of realizing the characters who already show those abilities.