In any case when you are brain dead, you just like having a long deep sleep.
When a person is in a coma, there is no indication how long you will be "sleeping".
Picture this scenario : you have been in a coma for week, months, years --- you show no sign of ever waking. The doctors/specialists have done all they could - every single medical test they could think of, every type of new medication/drug the could lay their hands on - none worked.
Are you going to allow the people around you --- your family, friends, the health care staff -- continue to look after you, with the hope that you may one day wake up? Well, what if you and you end up in a vegetative state? Does that not also present a situation alike when you were in a coma?
If that were to be the case, would you not allow your children to let you die?
I guess the best would be not to have children, then we don't have to put it on their shoulders to pull the plug or not.