Theoretical people
When performing utilitarian ethical calculus it is necessary to prioritize the happiness of the living over the unborn, the dying, and the people of the future. Quality of life for the currently living is the most important goal.
Concerns regarding the well-being of people who have not yet been born into the world are referred to as theoretical people, future humans, and other monikers.
Giving Them Due Consideration
That is not to say that future generations should not be considered, only that the living should not be asked to make sacrifices to their own happiness on their behalf if there is any way to avoid it. Any problem in society should be solved in a way that does not negatively impact quality of life for the living, and if it can't then people won't buy into it anyway.
The Effect of Altruism
Effective Altruism advocates for long-term planning and the creation of new technologies and fortunes in order save or improve the lives of large numbers of people in the future. However, part of the reason that altruism works is because of the emotional impact it has on the giver. Altruism is an emotionally selfish act, even if it is materially selfless. When you donate money to a cause, part of the reason for doing that is that it feels good to see the positive effects that the cause is having on real people. This emotional impact is significantly diminished when the people benefiting do not yet exist.
Effective Altruism ignores the Effect of Altruism on our emotions, and the relatively low ethical value of future humans in the moral calculus compared to the living that results. Therefore it is less effective and fails to achieve its own stated goals through this error.
What's Your Rush
Accelerationism is often used to justify the prioritization of future happiness. If the sacrifices we make now lead to the creation of technology like AI that can dramatically increase our quality of life, then the sooner we create those technologies the more people can benefit from them in the future.
Accelerationism ignores some fundamental aspects of human psychology. For example, the hedonic treadmill which suggests that no matter how cool our future tech is, it will become commonplace and won't likely have much impact on our baseline happiness.
New technology is more likely to have negative impacts when rollout speed is prioritized over social cost. The brain is not designed to handle constant change--it evolved in a world where the lessons learned as a child stayed relevant for your whole lifetime. Forcing people to change careers, move away from family, and easily fall through the cracks of the social safety net when things don't work out is incredibly stressful and a great source of modern suffering. Accelerating the pace of change without accommodating the real human needs that advanced civilization is supposed to serve will only increase suffering.
Then there is the cultural norm set by accelerationism that puts future generations over the present. When do we get to enjoy the fruits of our labor? When have we advanced technology to the point that we are generating the utilitarian value that we sacrificed for? We will always be able to conceive of more advanced technology that will make future lives even better than the present. So when do we get off the hamster wheel?
A society that maximizes happiness will ensure that the social impact of new technology is always mitigated by a strong social safety net. When new technology causes significant changes in the labor market, people should have ample time to seek new education and await good new job opportunities that don't require relocation.
Speculating on What Future People Will Be Like
Most fans of science fiction tend to idealize the "Star Trek" vision of the future of humanity, where we become ambassadors to the stars, seeking out new life and new civilizations, and generally living in harmony and abundance. Many of the other science fiction versions of the future tend toward the dystopian, so Star Trek stands out as one of the few that offers a more optimistic version without being unrealistically utopian.
Transhumanism advocates using technology, genetic engineering, artificial intelligence, and other means to transcend the limitations of the human brain and body in order to gain superhuman abilities or eternal life. Too often the advocates of transhumanism are willing to sacrifice the well-being of the regular humans in order to advance their dreams of advancement. This is bad utilitarianism.
Transhumanism also tends to jump the gun. For example, uploading your consciousness to the cloud is highly unlikely to result in a high-fidelity recreation, and may not replicate qualia at all. Yet, billionaires who have not bothered to come to terms with their mortality seem to think this might actually offer a viable solution within their lifetimes.
However, when we think on very large time scales like millennia, or even millions of years in the future, it is hared to imagine that there will not be some humans who choose to consciously evolve themselves, even if the majority choose to stay human. This political compass meme charts some of the ways that we envision our interstellar future with radically extended lifespans.
This video shows a likely scenario that billionaires who have frozen themselves cryogenically will face in the future, especially if current media trends continue..
Theoretical Future Music
The best depiction of theoretical future music in cinema is in Bill and Ted's Excellent Adventure, where a song they have yet to compose will one day bring world peace.