The average mammalian species lasts for about one million years. Homo sapiens have been around for only 200,000. With the benefit of technology and foresight, civilisation could, in principle, survive for at least as long as the earth is habitable — probably around 600–800 million years more.
Given that we can’t rule out this possibility, this means that there will, in expectation, be a huge number of future generations. There could also be a much larger number of people in each future generation, and their lives could be much better than ours.
We think future generations clearly matter, and impartial concern most likely implies their interests matter as much as anyone’s.