After sleeping on "The Reality War", I've come to a sort of verdict on it: I don't think this episode can be properly judged without acknowledging the behind-the-scenes issues that heavily impacted it.
Specifically, Gatwa's decision to step back due to the postponing of a formal renewal of the show.
This image has been kicking around. It's a promo image from "The Reality War" of a scene that isn't in the final episode.
It's known that reshoots for the S15 finale took place in February. Based on what the rumours and leaks say, the episode originally had a totally different ending, involving that dance party. I've also heard speculation that it would have set up Poppy as a plot hook for the next season, and there would have been a cameo from Susan Foreman to set up the plot about her return that RTD has been teasing from as early as "The Devil's Chord".
It's clear that RTD and co. were expecting the renewal to come much, much earlier than it did, and that waiting until after S15 had broadcast was not actually the original plan at all. Gatwa gets fed up because he doesn't want to put his career on hold like this, so RTD quickly concocts a new ending with a regeneration.
Poppy was never, ever intended to be turned into Belinda's daughter in the original ending, but whatever RTD had planned for her, he clearly felt it would only work with Gatwa, and so he hastily wrapped that up while also setting up a regeneration. (Incidentally, this also may have left answers to the Belinda/Mundy resemblance hanging.)
As for the Billie Piper of it all? The reshoots being almost last-minute meant RTD didn't really have time to do a full-blown casting for a new Doctor, and he clearly decided to go for shock and spectacle (for reasons that have yet to be determined). I suspect he decided on this rather than a mid-regeneration cliffhanger as a way to have a more decisive cliffhanger, hoping that people will want to know why this happened.
I will also add that Piper probably isn't playing the Doctor: no official source up to and including the episode credits have referred to her as such, and the regeneration itself also looks off.
Although RTD bears some of the responsibility for the messy way things went down, he doesn't own all of it since the lack of decision on a renewal was clearly the inciting incident here. I do hope that someday we learn what the original ending was ā it doubtless would have fit the story better. And it would have fit Belinda better as well.
At least we got Jodie Whittaker in a scene not written by Chris Chibnall. The way Thirteen is written there tells me that RTD hasn't completely lost his touch.
