suppose it depends on whether or not doctors can without any doubt or lack of certainty state that the person is like never going to wake up again.
if the odds are incredibly slim then obviously pulling the plug is probably the best choice for the individual and family. frankly often the person is probably already mentally 'dead' so.
but in instances in which there seems to be some hope obviously you have to do your best to try and honor the loved ones decisions and such prior to them going under. if they didn't make any indication of what they wanted then, again, I think it should come down to the odds. if they however did make it clear they'd want to be pulled along as long as possible if they was ANY chance of recovery then I say let them- as long as their funds or their loved ones funds can cover it to some degree
in terms of people being brain dead on life support for years on end- that's just an absurd concept and not something society can maintain though
