Mechanical ventilation is one of the many things that brings forward the question of when life ends. If you've got someone hooked up to a ventilator, and it breathes for them, but they are brain dead, are they really still alive? How brain dead do they need to be? Who has the right to make that call?
I don't really have the answers, but those are questions that doctors are having to struggle with at the moment. While mechanical ventilation has been around for decades, other therapies have caught up to the point where you could maintain a brain dead patient for a lifetime. But whether or not you should be doing this is not a question that is easy to answer.