[MikeCapone]

Not immortality, you can still get hit by a truck. Just indefinite lifespan.

At the rate at which fertility is dropping, and at the rate at which technology is progressing, it would probably be pretty sustainable. But even if it causes problems, these would probably be much smaller than the problem of aging we have right now; besides, we can't make that choice for the future. If people decide they want to die of aging, they can stop taking the therapies. But if they want to live, they'll have to figure how to make it work.

edit: I'll add that the main motivator of many people working on this is curing horrible diseases (cancer, alzheimer's, diabetes, etc), and living a really really long time is a side-effect of not becoming decrepit and frail.