We must respect not only patient autonomy, but the autonomy of all whose lives will be dramatically affected by the decision. On the mistaken assumption that only the patient is involved, the ethics of the doctor-patient dyad gives patients the right to make decisions about their treatment. The family is excluded by patient confidentiality from treatment decisions unless the patient wants them to be included. A genuine attempt to promote everyone's autonomy dictates that many treatment decisions should be family decisions. After all, that's the way people with families should make decisions that will dramatically alter the lives of members of their families.
Medical decisions are no different.
Sometimes, no doubt, family decisions are not possible, practical, or desirable. Even then, treatment decisions must attempt to maximize the autonomy of everyone whose life will be affected by the decision. If they do not, some people are inevitably marginalized or treated as a means merely. After all, frightened and seriously ill people may not be able to hold their own in difficult and emotional family conversations. So, for practical reasons, it may be best to continue to let the patient decide.