That's certainly not true for all Coursera courses. I took one in February - if I go back and look at it now all the material is gone (although the lecturer put the videos on youtube) and there is nothing but a flat description of what the course was. I'm actually signed up for a course now that I found out about half way through and didn't have time to do on the spot, so I'm looking for a way to download all the material because I fully expect it to disappear once the course 'finishes'.