To me it's a pretty simple matter... if T'Challa is gone, then after a certain period of time people would want Shuri to reclaim the role of BP. I think that's exactly what would happen ... hence it's what happened in the story. I'm a little curious about the exact limits of the BP mantle in the new Wakandan government too, but that's a seperate issue from whether or not the Wakandans would want a new Black Panther if T'Challa was no longer available. They would (IMO at least) and the obvious candidate would be Shuri.