Answer:
For example, if you are under age 40, have a frontal tumor, gross total removal of the tumor, are now high functioning, and have MGMT promoter methylation - these are all good. If you have some or all of these factors, your survival with additional treatment will be significantly longer than the estimated average you quoted.
If you have none of these good signs, then your question of whether to forgo therapy for better quality of remaining life is an excellent one.