She's become increasingly frail the last few years.
Yet some NYC millennial guy went to the gym with her and couldn't finish her gym routine.
Before this, I figured she would hang in until after 2020. Not so sure now. I was always expecting she would retire between 2020-2024. Even if she wanted to stay, by then, I don't think her health would let her.
If she retires in the next year, Trump could appoint pretty much anyone he wants, and the extra R seats on the senate will guarantee the appointment. Then I expect things to get sporty with the antifa, etc.
EDIT: I agree that she is a bad gambler. I suppose she thought Hillary was a lock, or at least "anybody but Trump". Given her age and time on the bench, the smart political tactic would have been to retire within the first year of Obama's second term. Garland would likely have been a shoe-in then.