But even for the meatspace courses in our uni, the course materials (lecture notes and exercise pdfs) remain available on the public course web page to the foreseeable future after the course.

Having predetermined time schedule might be reasonable for stuff like course credit and motivating the students to complete the course on time, but killing the forum community and even restricting access to materials (or making the access an unnecessary hassle) seems very unnecessary if the objective really is to foster learning in general.

Also you get community going through the course at the same time, can be useful for assignments where you grade each other's work, group projects, discussion boards, etc.