I don’t know how the TV contracts are tied together, but if no one is watching and the TV companies are required to broadcast WNBA games in order to broadcast NBA games, then they will pay the NBA less for the total package than they would otherwise. This because they have to pay broadcast personnel to be at the money losing WNBA games and need that money-losing requirement covered in the total package.
So that is how it becomes failure.
They’re not tied together. But they are both NBA products, so the NBA can arrange it however it would like.
And if the WNBA increases basketball interest among women, then the NBA may in part reap the benefits through NBA viewership and marketing agreements.