What if my child never graduates from college, and just keeps taking courses indefinitely?

You can protect against this scenario. You could, for example, put language in your agreement that support shall terminate upon attaining a baccalaureate degree, or the age of 24, whichever comes first. Since most children finish school by the age 22, the age 24 stipulation should suffice. It also is an encouragement to finish school on time and begin college after high school.