Define death though? These companies use the following analogy - it's 1950, you're walking down the street and suddenly you have a heart attack, drop to the floor and stop breathing. The ambulance arrives and says, 'yep, this person is dead'. If that happens today, they use CPR and in a lot of cases they resuscitate you. That's one example of how 'death' is a spectrum as opposed to a definitive state someone is in.
There's a difference between being 'fucked' and being 'dead'. If the same thing happened in the middle of the jungle, you're fucked and will most probably die, whereas if you're in a specialist unit at Wythenshawe hospital you're probably going to be alright.
The point you're making is basically saying that 'everyone who is officially signed off is beyond hope, even when considering future science'. Fortunately, you're most likely wrong. Not in all cases (if you're head is ran over by a truck and your brain splattered into a million pieces, yeah, you're probably screwed), but it's impossible to say that the brain is beyond repair in every case. Even with what you're talking about with 'agitated terminal delirium' - how do you know that by the year 3000 AD there won't be science available to cure the person? A cryonicist would say the more 'fucked' your brain is, the longer it will take science to be able to cure it'.
I think you're massively underestimating how advanced science will be in the future.
If you want to go into whether or not that person 'will really be that person', that's a whole different kettle of fish and one that it's impossible to talk about scientifically (with modern science).