But the reality of "parts replacement" in the present is most certainly more about practical healthcare factors like pain relief than about science-fictiony philosophizing. As the New York Times article quoted above states:
...osteoarthritis [can] wear down the cartilage in knees and leave sufferers with bone-on-bone rubbing and agonizing pain.Ouch! Sounds rather unpleasant. Fortunately for them, some folks (at least those living in wealthy, industrialized nations and able to access and afford health care) can find relief in total knee replacements, hip replacements, and other internally prosthetic surgeries.
Unfortunately, though (in addition to the lack of wider access to techniques that can have such a profound effect on a person's pain levels), the implants don't last forever. Knee replacement has actually been around since the early 1970s, and while materials and processes have certainly improved over the past three decades (newer replacement joints can probably last upwards of 20 years), the fact still remains that implanted materials such as titanium do not self-repair the way biological tissue can.
So, what I'm wondering regarding the long term is: will we end up developing better and better mechanical replacement parts, perhaps that can be maintained and repaired by the patient over time via injectable materials, or will stem cell and other therapies provide ways to restore ground-off cartilage and/or decrease inflammatory irritation such as that found in severe arthritis? I actually surmise that this is not an either/or prospect -- it is both likely and good that research will continue in multiple directions to help account for different people's individual situations, medical needs, and preferences.
Additionally, without even getting into the issue of what the "maximum possible human lifespan" might be, it does seem safe to note that (a) people in some countries do indeed live longer, on average, than their ancestors did, (b) the elder population is becoming more and more demographically significant, and (c) this trend has a decent chance of continuing in the future. It makes plenty of sense, then, to try and figure out more and better ways to help people stay mobile and pain-free well into several decades of living with a replacement part or two.
Again referencing the New York Times, doctors used to encourage people to wait as long as possible before getting a knee replacement, figuring that the patient would simply "die a natural death" before the part wore out. But some physicians are now re-thinking this advice in the advent of better and safer surgical techniques (that allow quicker recoveries) and in realizing that it doesn't make sense for people to live with terrible pain when there's a way to mitigate that pain.
Which leads me to my final point: while biomedical advances are certainly exciting to research, read about, and think about, even things like changes in physician attitudes can help a lot of people. Even with chronic illness, you can live to be 100, claims a recent MSNBC article -- and according to this article, one of the "secrets" to longevity is that of "Find[ing] a doctor willing to treat late-life symptoms aggressively". This seems like an obvious no-brainer to me: I mean, if you have diabetes or heart disease, why should the fact that you're 80 rather than 18 mean you get a lower standard of treatment by default?
And while I certainly look forward to some of the shiny healthcare advances that could be lingering just beyond the horizon (not just for my own future's sake, of course, but for the sake of family members and loved ones who are accumulating years), I definitely think there's plenty more that can be done now, using existing tools and resources, to help improve healthcare for the elderly and everyone else besides.