In 1930s mysteries, all sorts of motives were credible which aren't credible today, especially motives of preventing guilty sexual secrets from coming out. Nowadays, people sell their guilty sexual secrets.
I also hope that I am occasionally involved in projects that touch other people in ways that make their lives a little better, more interesting for the moment that makes them think.
You have to have something worth saying and then the ability to say it- writing's a double skill, really.