Pretty well-conceived scheme, as expected from Sakuya.
Marisa won't be leaving the manor anytime soon if she can't get the hakkero back.

Added to Heart-Breaking because this and the picture leading up to it have some pretty mean dialogues from Sakuya, and it's clear that Marisa doesn't win here. I was saddened when I read it, at least.

The way I see it, Sakuya's pretty much questioning Marisa's humanity and/self worth here.
Her plan was probably "Mess with her a little, wait till she got bored and then give it back"
But then Marisa kept at it, only growing desperate, like she only felt complete with magic, and it's just painful for Sakuya to see this girl she loves
having such a low image of herself.
So she put it in words.

Alternatively, Sakuya is trying to nudge Marisa to not be so reliant on magic. "Nobody would be concerned if you became unable to fly" sounds to me like Sakuya is saying that Marisa's value as a person isn't in her ability to fly, but rather in herself. That is, people wouldn't care if Marisa couldn't fly or use magic because they love her anyway.