Babies! I loved how they did the birth, at the river, and I am so happy for both Bev and Peter. Also, I'm amused that Father Thames went just as gaga as Mama Thames did. (I wonder if there is some sort of sympathetic magic there? After all, they are both spirits of the same river ...)

As to "why is fandom weird like that" I think there's a reason. I have seen several other cases, in my 20 years of online fandom, where people got upset that a supporting character isn't being treated like a main character. And in most cases, the supporting character is white and the main character is a person of color. When there isn't a racial aspect, people may still love that side character but they don't get offended when canon treats them like a secondary/supporting character.

If you're looking for other people to talk about Rivers of London with, there is a (very quiet) Dreamwidth community: http://the-folly.dreamwidth.org, and also a (somewhat quiet) discord: https://discord.gg/rdEFrKC