So, as with any recasting of the Doctor, there is a segment of the fandom who are displeased with the choice. The internet has been buzzing with pro-new doctor calling anti-new doctor people names and vice versa… Apparently if someone disagrees with your opinion you are automatically a snowflake. Let’s breakdown why someone would have a disagreeing opinion about the newest Dr.
Psychologically speaking, some people are comfortable with the way the show has been cast for the last 50+ years... and to change that makes them feel that what they internalized about the show and what they love will be taken away. Some others (it may overlap with some of the former as well... I can't do Venn diagrams in text... lol) feel that the decision was more of a stunt than an organic casting choice. Third camp feel it was lazy casting, as witnessed by the fact that Chibnall (who headed Broadchurch as well) just picked an actress he already worked with instead what would be the best choice for the new Doctor.
So, let’s explore some of these reasons. Being a member of a fandom means that someone has invested a lot of time and emotions into a character. With a character that has been around longer than 50 years, had 36 seasons, and 12 Doctors, there is a lot of room for people to internalize these characters. So, for someone to be upset that there is a change in the dynamic with a new incarnation of a character that they have spent time, money, and vested with some sense of emotional ownership over, is only natural. In fact, every time there is a new doctor, there is this argument. “Oh no, they will be a terrible Doctor.” I know several that didn’t like Matt Smith, and even more that didn’t like Peter Capaldi.