She's on her third season and since the previous two Doctors had three season contracts, it stands to reason hers is to. To say nothing of the "standard three seasons" for a Doctor is an idea that has been around since the classic era dating back to Patrick Troughton. Indeed, an interpretation of this was used as justification for dropping Colin Baker. There are very legitimate reasons to believe this is Whittaker's final season which have nothing to do with sexism and everything to do with established precedent.
It needs to be pointed out the modern era has conformed to the "precedent" by chance, though. Moffat almost talked Tennant into doing a fourth series, and Chibnall thought he had Capaldi for a fourth series right up until he didn't. Smith, on the other hand, left as soon as he could; as cool as I think his idea for a 2014 season with him and Tennant would have been, I wonder if he was entirely serious in pitching that idea to Moffat.
Hasn't Davison also said if that they'd had more stories like some of the ones in his second and third seasons he'd have stayed and that he made the decision to leave after 3 cos of stories like time flight?
Yeah. I don't understand all of the details, but Davison had to make the decision really early on, with a lot of stories still ahead of him. When he found he was enjoying the work more and the stories were better he had second thoughts, but by then it was already far too late. Well, it
wasn't, but JNT would have had to be a different person, and the BBC would have had to pay Colin Baker
not to appear. It was just easier to let Davison go and bring Colin Baker on board than change plans and retain Davison.
And in some of the post Companion Chronicles interviews Fraser Hines had said that Troughton had family pressure to move after 3 years.
The filming schedule in the 60s was grueling, and Tat Wood describes the filming in 1969 in
About Time 2 as something Troughton, Hines, and Padbury endured to fill the week between their weekend partying and carousing. So it was probably some of both -- the filming took a toll, and there were issues at home as a consequence of
Doctor Who.