For instance, I see nothing desirable about living much beyond 80 if I'll be senile, or living in a nursing home, or suffering from the myriad of conditions the elderly typically do. It's not enough to simply survive to an old age, you have to be able to enjoy it and do the things you like doing. Without that, it's not really life anymore, not the way I understand it or want it, so extending it beyond those limits would not be desirable or beneficial.

As for it being moral, that's simply ridiculous. Very few things we do today are in any way "natural," and in any case, there is no one to judge what is natural and what isn't. Overpopulation and resource scarcity are problems, but they're not insurmountable. If human longevity is rising at such a rapid pace, then it's only because human technology is advancing even faster, so I'm confident solutions can and will be found. And if not people will simply starve and die and the problem will still resolve itself.