Flex studying should be possible, but under certain conditions. Whenever someone for example only has to follow less than a half semester's worth of courses, it should be possible to pay per course. This way a reasonable balance between the amount of courses and the financial obligations bound to those courses is maintained. You also limit a number of practical concerns: students can not randomly follow courses without having to have passed several earlier courses. Furthermore, the possibility of having five students for a course in one year, and five hundred in the next year, is smaller if you set some conditions for flex studying.