I find this argument incoherent, as I reject the idea of a person at the age of 1 being the same person as they are at the age of 800 - or for that manner, the idea of a person at the age of 400 being the same person as they are at the age of 401. In fact, I reject the idea of personal continuity in the first place, at least when looking at "fairness" at such an abstract level. I am not the same person as I was a minute ago, and indeed there are no persons at all, only experience-moments. Therefore there's no inherent difference in whether someone lives 800 years or ten people live 80 years. Both have 800 years worth of experience-moments.

I do recognize that "fairness" is still a useful abstraction on a societal level, as humans will experience feelings of resentment towards conditions which they perceive as unfair, as inequal outcomes are often associated with lower overall utility, and so forth. But even then, "fairness" is still just a theoretical fiction that's useful for maximizing utility, not something that would have actual moral relevance by itself.

As for the diminishing marginal returns argument, it seems inapplicable. If we're talking about the utility of a life (or a life-year), then the relevant variable would probably be something like happiness, but research on the topic has found age to be unrelated to happiness (see e.g. here), so each year seems to produce roughly the same amount of utility. Thus the marginal returns do not diminish.

Actually, that's only true if we ignore the resources needed to support a person. Childhood and old age are the two periods where people don't manage on their own, and need to be cared for by others. Thus, on a (utility)/(resources invested) basis, childhood and old age produce lower returns. Now life extension would eliminate age-related decline in health, so old people would cease to require more resources. And if people had fewer children, we'd need to invest fewer resources on them as well. So with life extension the marginal returns would be higher than with no life extension. Not only would the average life-year be as good as in the case with no life extension, we could support a larger population, so there would be many more life-years.

One could also make the argument that even if life extension wouldn't reduce the average amount of resources we'd need to support a person, it would still lead to increased population growth. Global trends currently show declining population growth all over the world. Developed countries will be the first ones to have their population drastically reduced (Japan's population began to decrease in 2005), but current projections seem to estimate that the developing world will follow eventually. Sans life extension, the future could easily be one of small populations and small families. With life extension, the future could still be one of small families, but it could be one of much larger populations as population growth would continue regardless. Instead of a planetary population of one billion people living to 80 each, we might have a planetary population of one hundred billion people living to 800 each. That would be no worse than no life extension on the fairness criteria, and much better on the experience-moments criteria.