Theatre Ideas: Outrageous Fortune: Chapter 1: Build a Bridge, Don't Take Swimming Lessons:
Because, again, if you look back at theatre history, you see that this problem has already been solved. The Lord Chamberlain's Men didn't only produce Shakespeare, they had a relationship with many playwrights. Furthermore, Shakespeare wasn't only a playwright, but also an actor and a shareholder in the theatre. Moliere's troupe did Moliere's plays, yes, but also the plays of other playwrights (such as Jean Racine), who they also nurtured. The Moscow Art Theatre didn't only do Chekhov's plays, but many others as well.
But these theatres did a lot of productions, so they needed a lot of plays. As long as regional theatres only do a handful of productions each year, and only a small percentage of those are devoted to new plays, then the worried artistic director quoted above is right to be concerned. The answer, however, is to do more plays.