I'm guessing her desire to leave is more an expression of her need for autonomy. I wouldn't be surprised if she started working on her escape plan as soon as you "refused to accept" that life for her, but waited until she had a safety net to let you know about it.
It's unlikely that you would go from completely dismissing the possibility of polyamory to being completely OK with it overnight, despite the external motivation. It sounds like her reasons for leaving you are more than just your insistence on monogamy.
Are you not concerned about her desire to be "provided for" even if it means forgoing happiness? Despite any good news you have on the horizon, are you not concerned that she would be staying with you just for your money?
I suppose there's no harm in proposing it and seeing what she says, but looking at the big picture, I don't anticipate this approach leading to a positive outcome.
“As I am sure any cat owner will be able to tell you,
someone else putting you in a box is entirely different
from getting into a box yourself.” —bisexualbaker