DVD sales are a pittance next box office, especially in today's market.
For example the movie Titanic. One of the biggest movies ever in history and one that came out in the height of the VHS / DVD era. Lifetime its brought in a grand total of about $33 million in VHS / DVD / Blue Ray sales. And thats before you take out store cut and production costs.
DVD sales and cable can popularize a movie after release and thus get a movie that flopped first time around a sequel. Austin Powers is a good example. But they won't ever be enough to make or break a film. Especially now. If Titanic only made $33 million how much do you think a movie that comes out today in the streaming era makes?