When Calle was announced as Supergirl, Routh and Benoist congratulated her but Cavill didn’t. He might want to leave her out, we’ll see, but Flash apparently ends with Cavill and Calle together greeting Barry. That’s an easy segue way into the next Cavill Superman movie featuring her if he wants to. If she’s really as good in the role as leakers think it might be a good idea to have someone for Cavill to bounce off of, because Lord knows he’s not a strong solo act.
But I agree that ideally it would be better if the movie focused just on Cavill. Johns Brainiac arc which I’m certain the next movie will take inspiration from already featured Supergirl in a secondary role. Be simple to just use that.