Theo: This is in my opinion the biggest hole in Epicurus for modern times. Back then E. was telling people that death was nothing because he wanted to erase the fear of an afterlife of eternal damnation. fast forward today in a world much more secular than the ancient, fear of death for many people has nothing to do with fear of a nightmarish afterlife but simply the end of enjoying things they like to do. Let me give an example. let's say that I'm getting the highest of pleasure from space exploration. I have no concerns about afterlife and I don't care about money, fame and the likes; I just want to travel space star trek style. But to enjoy that I know I must wait couple of centuries of technological advancement which under current situation is impossible. my certain death will deprive me from the thing I get the most of pleasure. the point is that in E. times space exploration was not a prospect for humans (as many other things); today it is and death is an unwanted barrier.
Theo I think at least one response to your point is that people have always wanted more than they have the ability to obtain, and death has as always been a barrier to that. "Death is nothing to us" has little relevance to that problem. As I understand the Epicurean doctrines that ARE relevant to that problem (such as PD 18-21), we should use "true reasoning" to understand that the body has no need of more pleasure than it can experience when it is functioning well and without pain. That condition of the body is relatively easy for most people to obtain, and new pleasure for the body in the future is simply variation of the same pleasures in the past. Those new pleasures (eating lunch tomorrow) are desirable, but not possible to extend to forever due to our limited lifespan. But the body doesn't know or care about the future.
The mind, however, can construct limitless desires such as your desire for space exploration. The answer to that problem comes only with the study of nature and the realization that the body and mind are mortal, with limited lifespan, and neither will survive death. Since the guide of life is pleasure (and not the quest for unlimited knowledge) we can consider the problem and realize that filling our experience with pleasures is not generally so hard either (such as Lucretius' observation of lounging with friends by the river). Repeating pleasurable experiences day after day is variation of the same pleasures, which is in fact desirable, so we want to stay alive as long as we can continue that.
But we know that we will one day die and our time will be up. That isn't a "hole" in Epicurean theory, that is a recognition of fact of limited lifespan that no theory can override. Applying Epicurean observations and philosophy shows us the path to the living the best life possible to us, and if we steer for that then we should have no anxiety that we could be doing anything better or higher. Those frustrations are frequently planted by false religions and false standards of absolutes/rationalism/idealism, or simply by naive absence of thought about the nature of human life.
So from this perspective I don't think your space illustration indicates that anything has changed. You are simply looking at a desire which has no limitation (you can always go further and further into space, tomorrow and tomorrow and tomorrow) and you are seeing that that is impossible due to limited lifespan. You can label that "unnatural and unnecessary" if you think that helps, but the bottom line is that Epicurean philosophy doesn't promise the impossible. No philosophy or religion can deliver the impossible. All Epicurean philosophy can do is point out to you that yearning for the impossible just increases your anxieties needlessly, because "worrying" about it isn't going to change anything.
I can't end this post, however, without pointing out that this isn't a justification for sitting in your cave eating bread and water. Sitting in your cave doesn't guarantee a thing except that you will reduce the total experience of pleasure that might otherwise have been possible to you had you engaged reality more aggressively. And if pleasurable living over the lifetime that nature makes available to you is your goal, as it should be, then you are ignoring and perverting that goal by choosing to sit in a cave when happier alternatives are available. And for those who want to focus on the fact that sometimes happier alternatives are not always available, I ask that you please cite your authority, in Epicurus or otherwise, that "life is fair."