Prince Prospero still won,. Francesca was corrupted.
The seeds of depravity he had planted clearly took root in her as illustrated by her placid acceptance of the burning murder, her declaration she gave herself to Prospero (and by extension Satan), and the kiss to Prospero as she leaves the Masque.
Poe should have done a sequel of her life with the boyscout. I don't believe he would have lasted long before she found him too boring. She would need to seek elsewhere for someone more stimulating as per her new outlook. If he needed to be eliminated for this to happen, she now could like with it.
Put another way, how you gonna keep them down on the farm after they've seen Paree.