To begin, let us imagine that we have what appears to be a perfectly normal human, let's call him Todd. Todd was born and grew up into adulthood under fairly unremarkable circumstances. But there is one remarkable fact about Todd: he appears to have an indefinite lifespan. More particularly, he remains in a continuous state of early adulthood for centuries or perhaps millennia on end. But current medical science can seem to find no explanation for why he seems to possess eternal youth. Indeed, for all we know, Todd may well be immortal.
On the other hand, it may very well be that Todd will indeed die of 'old age' at some indeterminate point in the future, for he could just be undergoing the exact same aging process, just at a much slower rate than the average human. So, with all the best evidence we have at our disposal, it is indeterminate whether Todd is really immortal.
Now with all that being said, we must ask ourselves: does Todd's life have meaning? Perhaps it is not fair to pose the question in such a broad manner, so let us be more specific. For any age T, where T is an age far greater than the currently longest-lived humans, does Todd's life have meaning at T? Suppose T is 200 years old. Is Todd's life meaningful at T? Maybe the great majority of us would be willing to concede that it is.
So let us now increase T to something like 500 years old. Is Todd's life still meaningful? If the answer is still 'Yes', then let us increase T yet again. If we keep following this procedure, one of either two outcomes will take place: Either we will change our answer to 'No' at some sufficiently high T, or we will always answer 'Yes', no matter how high T is.
Let us consider the first possibility. If we do switch our minds at some sufficiently high T, then we would be suggesting that T is the limit age for a meaningful life. So if Todd is closely approaching T, then he will soon be faced with living an indefinitely long life of complete and utter meaninglessness. What should he do when faced with this information? Should he perhaps take the drastic measure of committing suicide just when T arrives, thereby assuring that he has lived a meaningful life? But how can it be required of someone to commit suicide to ensure that they have lived a meaningful life?
If he doesn't go down this road, then should he just fall into despair and resign himself to a life of meaninglessness, even if there will surely be an endless variety of potential endeavors to which he could apply himself? But the most important question of all here is: how exactly can we non-arbitrarily determine the limit age for a meaningful life?
But let us suppose that we don't go down this road, and that we will always answer 'Yes' when asked if Todd's life is meaningful at any arbitrarily high T. If this is the path we take, then we have literally said nothing else than that an indefinitely long life can be meaningful, and thus we have rejected the notion that death is necessary for a meaningful life. So those who defend the idea that death is needed for a life to be meaningful seem to be stuck between a rock and a hard place.
We might also add that if an indefinitely long life is a meaningless life, then it follows that AI programs cannot have meaningful lives (if we use the term 'life' in this context to mean something like 'existence' or 'conscious experience', since AI programs clearly don't have biological lives). Since AI programs can be implemented on a variety of physical devices and since copies of them can be easily produced, they quite literally have indefinitely long lives. Should we then say that their lives are doomed to be meaningless? But surely AI programs, especially as physically implemented in robots, can engage in a multitude of meaningful endeavors. The simple fact that such meaningful endeavors do not have a foreseeable end doesn't seem at first glance to automatically write off the possibility that their lives can be meaningful.
Somewhat related to this point, if mind uploading becomes a viable possibility in the near future, then human minds will be in the exact same situation as AI programs. For once we have the ability to upload our minds onto computers and to make backup copies of them, then our lives will then be indefinitely long. Should we then say that our lives can only remain meaningful if our minds are encoded on biological brains? But why should biology be so closely connected to a meaningful life?
So to conclude, I don't view the notion that death is necessary for a meaningful life as being self-evident. In essence, what I would like to see in defense of this notion is at least something by way of decent argument. But quite apart from that, I would like to see more discussion on possible ways to live an indefinitely long, yet meaningful life.