I think she threatens to leave because she knows you won’t let that happen.

She can’t leave, she can’t support herself and can’t even handle daily tasks, even if you pay her alimony it’s typically not enough to survive on her own unless she wants to live in extreme poverty, and she has no extended family to help her.

So she ain’t going no where, at least not on her own accord. She just manipulates you

I’d call her bluff. “Sure I understand you want to leave. That’s ok. Let’s do that.”