Deathism is a label of convenience for any philosophy or outlook that regards death as a good thing. These worldviews also tend to be in favor of both degenerative aging and the involuntary nature of death - that we are forced to die regardless of what we might think on the subject. A deathist is someone who holds such a viewpoint. One of the more sensible comments I've seen on deathism in general is this:
Such is human nature, that if we were all hit on the head with a baseball bat once a week, philosophers would soon discover many amazing benefits of being hit on the head with a baseball bat: It toughens us, renders us less fearful of lesser pains, makes bat-free days all the sweeter. But if people are not currently being hit with baseball bats, they will not volunteer for it. Modern literature about [death and the prospects for radical life extension through medical science] is written primarily by authors who expect to die, and their grapes are accordingly sour.
One of the hoary old arguments put out by near everyone in favor of unavoidable death is that "death gives life meaning." The conceit here is that life is somehow meaningless until you can draw a line under it and assess, or perhaps that no-one would do anything if they didn't have a timer counting down their own personal extinction. I've never been able to grasp the essence of the first point, which just seems so much nonsense to me: why draw the line on death? Why not somewhere else? The past at any point is fixed and up for evaluation, but why draw lines at all for that matter?
The second point can be thrown out on the grounds that humans with an adult life expectancy of 80 behave remarkably similarly to humans with an adult life expectancy of 40-something, as any exploratory expedition through the better-recorded sections of Roman history will demonstrate. Where differences exist they certainly don't involve people lazying around as the expectation of additional years grows, but are rather changes in the nature of the tasks that people busy themselves with. A longer time horizon means that you can undertake better, more ambitious, more profitable projects by virtue of having longer in which to complete them. Competition if nothing else drives that process.
Since various deathists persist in arguing that involuntary death (without or without the suffering and pain of aging) is necessary to give life meaning, there is a steady flow of articles from the radical life extension advocacy community to point out just how ridiculous the deathist position is. Here is one of the more recent examples:
One common argument against indefinite lifespans is that a definitive limit to one's life - that is, death - provides some essential baseline reference, and that it is only in contrast to this limiting factor that life has any meaning at all. In this article I refute the argument's underlying premises, and then argue that even if such premises were taken as true, its conclusion - that eradicating death would negate the "limiting factor" that legitimizes life - is also invalid, because the ever-changing state of self and of world can constitute such a limiting factor just as well as death can, which can be seen lucidly in the simple fact that opportunities once here are now gone, and that it is not death but life itself that is responsible for that.
Culture is in constant upheaval, with new opportunity's opening up(ward) all the time. Thus the changing state of culture and humanity's upheaved hump through time could act as this "limiting factor" just as well as death or the changing self could. What is available today may be gone tomorrow. We've missed our chance to see the Roman Empire at its highest point, to witness the first Moon landing, to pioneer a new idea now old. Opportunities appear and vanish all the time.
Indeed, these last two points - that the changing state of self and society, together or singly, could constitute such a limiting factor just as effectively as death could - serve to undermine another common argument against the desirability of limitless life: boredom. Too often is this rather baseless claim bandied about as a reason to forestall indefinitely-extended lifespans - that longer life will lead to increased boredom. That self and society are in a constant state of change means that boredom should become increasingly harder to maintain.