1) Reduced decline: 80 years of vigor, 15 years of decline
2) Fixed decline: 100 years of vigor, 20 years of decline
3) Subproportional decline: 120 years of vigor, 30 years of decline
4) Proportional decline: 180 years of vigor, 60 years of decline
5) Superproportional decline: 240 years of vigor, 120 years of decline
You have come up with a 6th option, crappy extension: 60 years of vigor, 200 years of decline. Well, maybe, but why do you suspect that bio-hacking is going to turn out to be so very disappointing?
One possibility is that before we get there, we might go through a phase of relatively crappy extension where we tend to end up stuck at a plateau somewhere towards the end of the decline stage. What I'm basing this on is the fact that (according to Michael Rose) fruit flies and other organisms reach a senescence plateau where their rate of death stops increasing with time, instead remaining at some fixed percentage rate per hour death rate stays constant. In biological terms that means they aren't aging any more, i.e. the death rate is not going up. He claims this happens in people too, just at a very late stage in the life cycle.
The idea I am going for is that idealized life support systems, robotic exosuits, implantable organs, and so forth could act as a crude counterbalance for the death probability (taking it to near zero) without actually altering the fact that your body has aged (i.e. would die very soon on average without the external life support).
IMHO living for centuries with the body of a 90 year old wouldn't be so bad. (Supposedly there's a point where the body stops getting any more frail.) And as long as you can keep mortality risk to a low enough level for long enough, it's just a matter of time before youthfulness-restoring tech comes along.
I'm sure you've seen http://gravityandlevity.wordpress.com/2009/07/08/your-body-w... already, so you know you can't live for centuries with the body of a 90 year old because the decay and mortality is just too bad, fancy exosuits or no.
So this leads to the kind of odd thought experiment which runs more accurately like 'imagine you have the organs and general health of a 12 year old, but are weak and debilitated and functioning at a low level (though not getting any worse); would you still find life worth living today?'
I'd say the answer is yes. Elderly suicide is more frequent than among the middle-aged or younger, so life isn't as good (not a surprising claim), but suicide is still rare, so life is still somewhat good.