Here's how I made the decision. I ran across this little exercise, which was really clarifying for me:
Since you can't know with any certainty the long-term effects of your decision, focus for a moment on the short-term: Close your eyes and imagine you've decided to leave. Right now, without thinking of what might or might not happen, how do you feel? Now imagine you've decided to stay. How do you feel?
When I imagined leaving, I felt an overwhelming relief at not having to read all the back issues of academic journals that were piling up on my bookshelves. It seems like such a small thing -- but I made the decision to leave that day, and I've never really regretted it. I miss certain things about academia -- but I'm happier and have more options where I am now.