In fact we are arguably a very important chapter in that story as not only are we the first (known) species on this planet to achieve highly developed concepts and traits that are shared by many other lifeforms on this planet such as communication, technology, emotion, etc. but we are also the first species to arise that now effectively controls its own destiny often in defiance of more basic universal influences on biological life such as natural selection.
We are also pretty special in that we have now reached the point (just about anyway) where the creation of new life by ourselves is no longer limited solely to biological procreation. It looks likely that in the near future we will be capable of creating new life of one new kind or another. This would be either through modification of the current human 'original' species or by building new biological entities from scratch using the more fundamental chemical building blocks that life on our world currently operates from. Perhaps more dramatically from a universal viewpoint we will also be capable of creating life that is totally non-biological.
Another distinguishing feature of our species is that we are spiritual and capable of reciprocal spirituality. Although that word has as many different meanings as there are people on the planet it is none the less a species-specific characteristic unique to humanity as far as we know. We have the capacity to conceptualise spiritual ideas and to respond (or not) accordingly to them irrespective of their origins.
Spirituality has proven to be such a universal trait in human society and culture down through the millennia that I think that not only would it be of interest but perhaps of necessity for any future human-derived lifeforms to experience and appreciate it. And if our sense of spirituality does indeed connect us with something beyond the physicality of our immediate universe than it would be unethical to deny that to any of our creations if they possessed the capacity to appreciate it and connect with it as well.
I'm currently reading a new book which features an essay by David Wilkinson, a theologian from Durham on the Biblical understanding of creation and humanity's place in it. He suggests that the image of God in humanity
"is that part of creation that is capable of being concious of and responsive in its relationship to the Creator."which enables both continuity with the rest of creation as well as a God-given uniqueness. If this is the case and other new human-derived beings also reflected this fully as beings derived through ourselves, our culture (and having the freedom to not be limited to either) and spiritual relationship would it be necessary for the original human creators to survive in order for the New Creation and return of Christ as both envisioned by Christianity to come to pass?
Although I have already said that I would like us to I don't think it is necessary as we will have created true successors, a new form of children, in every sense of the word. The only difference would be the method of replication used. We already except this in a small way today with some children fertilised outside the womb and then implanted and allowed to mature naturally inside a woman. This change in reproduction, or rather additional ways of reproducing, might not be a single giant step that creates a radical difference between us and 'them' but rather it could happen through a variety and a continuum of different methods bridging the many extremes.
My personal hope is that all would find a place in an expanded society. It is likely that one of the drivers for creating these derived life forms will be the expansion of humanity into previously inaccessible environments such as the deep oceans, other planets or space itself. Indeed non-biological life is likely to be the first life from our world to reach the stars. They will be tomorrow's ambassadors and explorers for our society to the universe. If this is the case I would argue that there is plenty of room for all and no reason why humans should necessarily be replaced entirely or inevitably either.
From a Christian point of view the Bible talks of a restored creation at the return of Christ which we all look forward to. Some would argue that this precludes the extinction of biological humanity before the return of Christ. I would however disagree. I think is reading too much into a text written by and for a people to whom these newer concepts would have no meaning. We must look at the wider message of the Bible and creation to think about these ideas rather than over literallising specific verses or passages with a 21st century viewpoint.
If these new creations are our children then they would also be created in the image of God just as we are. Being created in the image of God is a massive concept that theologians have wrestled with for centuries, but an important point to note on it is that it is less tied to physical appearance (as God is spirit) and more to do with delegated responsibility, representation and stewardship. If our creations share this component of our 'essence' they would fully capable (and willing!) of continuing the Christian mission and legacy independent of ourselves. Indeed, in their own niche locations they may continue and expand it without any contact with original humanity.
In fact it would be quite interesting to see how such new life would discuss and develop their understandings of theology both in conjunction with and independent of ourselves. Given the breadth of interpretation on theology in the last 4000 odd years I think we'd come across some big surprises. (For example our current intergalactic understanding of the universe is completely different to the ancient Hebrew view of the cosmos and the understanding of scripture is appropriately recast in the journey from the Hebrew viewpoint to our current one.)
So, are we necessary? For the moment and for the foreseeable future - yes. Must we survive? No. That however is not something to be shunned or feared. In fact if our species does its job well, we would be both valued if we survive and mourned by our descendants if we don't.