I think its possible to extend life, by doing spot repairs and attacking specific problems and conditions. But I don't believe the human body was designed by evolution to live forever.

It would have no survival value for the species as a whole. DNA needs to be transmitted, it doesn't need to be delivered intact when it can make copies of itself and accomplish the same thing.
Any immortal organism would compete for mates with the mortal, which would slow down evolution.

Death by natural causes isn't a disease we can vaccinate against. There is no single flaw which can be corrected to defeat it it. It's more like erosion or corrosion. You can take maintenance steps to minimize it, even partially reverse it, but in the end, rust never sleeps.

I believe human life extension will succeed in doubling or perhaps tripling the life span, but it will be complex and expensive and limited only to those who can afford it.

Remember, we have not really succeeded in increasing the human life span. People have always died off eventually with a max cutoff around a hundred or a little more. Our AVERAGE life span has increased enormously, because children don't die routinely any more, and we know how to prevent or cure many natural killers with hygiene or medicine or nutrition. But the average age of death for the healthy, somewhere near the biblical three score and 10, is still around the same