The theory that in one point in time, science will help us live longer say a couple of years, and during that time, science will find out how to make us live some more years etc until we can live how long time we want to.
I really don't get the debate against even just living longer...
The point of longevity isn't to have a rambling grape, it's to have people stay in their prime for significantly longer time. This means anything you could want to do then would be avaliable. With optimal mind too, imagine the spike in development of technology. Maybe the singularity or something, and all of a sudden anything anyone imagines can be done. It'll be a world of dreams, and surely dreams can't get boring, right?