I have a lot of respect for DC in keeping faith in the Movement even if the audience was limited and sales slow, because they wanted it to find that audience. We're always told that sales are key to keeping books going. Which is why it's so confusing that

Books are not always canceled for sales reasons, other things come into play, as well. If a superstar creator wants to work on a book and it will lead to huge sales, they will often end a book before that person comes aboard. 

I have no insider knowledge about Supergirl, but knowing what I know about the industry, I am almost positive a new book is on the way.