I mean, where do you think the light goes when previous Princesses of Heart pass away? It's definitely been a few lifetimes since the Keyblade War. Even then, Xehanort has mentioned in the reports in BBS that Keyblade wielders outside their circle, that is aside from Eraqus, Xehanort, Yen Sid, and their disciples, do exist. Even without Kairi, all they really need is one extra light.

Hell, they could also use the Princesses to force the Keyblade wielders into fighting them through torture rather than killing. Killing Kairi was a good motivator for Sora, who was right there, but torturing the Princesses and showing the guardians of light would also be a good means of provoking them. It'd be kind of like what Xemnas said but to a lesser degree. They need the Princesses' hearts, but it doesn't really matter what their physical condition is like just so long as they can still clash. Even with so much at stake, there's no way Sora would be able to hold back knowing that their inaction is directly leading to the harming of others.