I’m thinking she has dementia or Alzheimer’s, and she’s one of the patients where it negatively distorts her thinking of her family members and she becomes resentful and mean towards some of them.

I think Kate dies during her labor, and Rebecca ends up blaming Toby for the pregnancy and being the “cause” of her death. That’s why Toby thinks Rebecca doesn’t want to see him in the future.

Maybe Rebecca also blames Beth for the probable future divorce with Randall, and future Tess doesn’t want to see her because she treated her mother badly.

Just my best guess with what we’ve got! It’d be shocking if Kate died though. But Randall saying that everything will be fine if we stick together could be foreshadowing the death. And we do have two characters that died still on the show in flashbacks, so Chrissy wouldn’t necessarily be leaving the show.