If you cannot find a meaning in one lifetime, adding a finite number of (other people’s) lifetimes will still be meaningless. (It can make the ending of your own less relevant but I’d say that’s a different issue.)
I think grasping this is a fundamental step towards escaping a very egocentric view of life.
If whatever you can do for today’s people is not enough to have a “meaning” how is doing things for future people different in that respect?
(Again, there is a practical difference in delaying the meaning beyond the time of death. It’s a common trait in religions for a reason, I guess.)
Whether this idea helps to find a meaning in life or not is debatable. If imaginary people from the distant future can give meaning to one’s life also should actual people from the present/near future, at least to some extent. Otherwise we’re fooling ourselves until the end.
This may be true for ants. But I've never seen how this can be an absolute truth for humans for two reasons:
1. If one life is truly meaningless (0), how does multiplying it produce meaning (0 * 8 billion = 0)?
2. If you found yourself to be the last person on the planet, would you A) realize you have no meaning and die, B) create art and put a little more beauty into the universe? Given those choices, does option A or B have more inherent "meaning"?