How to prepare academically-minded students and postdocs for the eventuality of not finding secure academic employment?
A few of your students may just study your subject for the pure love of it. More of them, I suspect, will settle for nothing but an academic career. You may not be able to convince them to aim otherwise. 

But, you can still, with some effort, show them all what the alternatives are. In particular you can do two things, one simple and the other harder. 

The first is to develop a list of sources in various industries that utilize the sorts of things you teach. Invite someone to visit your class about one day a month and to talk about the sorts of applications that are done in their industry. Even better if they can talk about their own personal involvement. It will take a while to get started in this, but a contact might be willing to return annually. You can expand this, possibly, if there are several sections of your course, even those taught by others, and have a late afternoon or evening seminar with such a guest. 

The second thing is to develop, over time, an internship program with local industries, where students visit the companies and maybe perform some tasks there. But just visiting the companies to see what life might be like in industry might be enough. Many universities have internships and some depend on them as a fundamental part of the education, so you might be able to get some help in this. 

One of the problems of the future academic mindset is lack of knowledge of the alternatives. You can give them at least a hint of that.