First of all, it is not “considered impossible to extend human lifespans” by everyone. Nevertheless, in general, it is considered impossible to extend human lifespans because that is the way we are conditioned to think.
Science and engineering are the key to extending human lifespan, but they have only been around a couple of centuries or so and have not gained acceptance by the general public. The masses turn to religion and superstition to assuage their fear of the horror known as death. They send their money to TV evangelists who promise them salvation via an unproven package of lies that appeals to their fear of death and desire for meaning in life.
This lie extends all the way through the people of political power and those who control the money that could extend our lives. And, even to scientists who maintain religious beliefs despite their scientific training. The human animal is very susceptible to mysticism and as soon as this kind of thinking disappears the sooner we will get down to the task of eliminating the suffering of aging and death.
If you would like to extend your healthspan (healthy lifespan) another 10 or 20 years, upvote and resteem this post.
