Had the Dems taken the Senate, RGB would only have to make it to Jan of 2020 in order to give a chance at a Dem nominee. But Republicans picked up seats, so she would need to last until at least 2021 and with Trump's popularity she would likely need to last until 2022 or 2024. Due to her age and health RGB has almost no chance of outliving Trump's time in office. She will give Trump a replacement, it's just a matter of when. Knowing that might convince her to step aside.