; Date: Sat Jan 02 2021
Change is in the core structure of Doctor Who. Every actor, the production team, the TARDIS interior, the locations, everything can change, and it is still Doctor Who in every episode. There have been dozens of Companion Departures in Doctor Who, so there's nothing new about this.
Apparently both Bradley Walsh and Tosin Cole have other roles to pick up. It is common for Companions to last for two seasons.
What it may mean is that the BBC is gearing up to replace Whittaker with a new actor. One pattern that sometimes happens is that for a given Doctor's last season, the Companions are changed with the Companions carrying over to a new Doctor. For example, Matt Smith's era was defined by his relationship with Amy and Rory, but they left for Smith's last season to be replaced by Clara, who in turn defined Capaldi's time as The Doctor.
The Wikipedia has a long explanation of the new guy. Basically he's got a long career of appearances on television including as a comedian, and prior to that he played professional soccer. I've watched a couple clips on YouTube and he can sure carry himself. But his presentation is of a brash British person who knows how to deliver funny lines, and it's hard to see how that'll translate as a TARDIS Companion.
Doctor Who is not a sitcom. What is the BBC thinking?
Many fan pieces are complaining that the character - Dan - is going to be yet another "modern human" Companion, and why can't we have Companions who are from other time periods or other planets? The history of Doctor Who is full of such people, but why are the vast majority of Modern Who companions plucked from modern-day Great Britain?
On the other hand there's this:
Okay, that fits the modern times, a Doctor Who actor testing COVID-Positive. But he's been filming on the set for "a few months". How does this affect the Doctor Who production schedule?
Also, that he's been filming for "a few months" indicates the new season episodes are hopefully well along? And we'll hopefully see new episodes shortly rather than later?
Many of the previous Companion departures were abrupt things. For example when Sarah Jane Smith had gotten upset and packed up her things to leave, only to be told by The Doctor that he'd been abruptly summoned to Gallifrey and she had to leave anyway. He was supposed to leave her someplace convenient, but instead dumped her on the wrong side of London.
In this departure, there was careful consideration by the characters, and a group hug.