but even with hard-real-time work, most of the situations allow you to miss a deadline once in a while. If your production line has to be stopped daily or hourly, you can't accept it, but if it stops once a year because the deadline was missed, that's probably acceptable.
you could also have a hard-real-time task that has a 5 second deadline to get the task done. non-real-time linux can accomplish that today (except in the face of failing hardware)
even things with fairly short deadlines can be made statistically reliable in many cases. It's only when you start getting into extremely short deadlines or systems that also handle non-critical processes that compete for resources that you start having real problems.