Intentionally being provocative here, but by that logic, why does your effect on other people matter? You are unlikely to leave a lasting legacy, and the generation you do affect, will die as well.
By this logic, culture and society would die every generation, and have to be rebuilt from scratch each time.
We all leave behind a "small" but far-reaching legacy that ripples out from our short lifetime. Each of the thousands of interactions we have with with other people and our general environment have a tiny but real impact that doesn't necessarily diminish to zero after we die. The change that occurs then has a small domino effect on any other person or system that it touches. And so on and so forth :)
My life today is deeply affected by the concerted actions of billions of unknown individuals from centuries and millennia past in ways that I can't even begin to fathom. I'm grateful for some of those impacts. For other impacts less so, but I hope to contribute small changes for the benefit those who live in the untold distant future.
Even to pick a small part of it, 130,000 people worked on the Manhattan Project but a history of it that the average person would consume might name 10 key figures.
To use your example of the Manhattan project: only 10 people may be remembered in books, but they certainly would have never completed the project by themselves. The contributions of those other thousands of individuals was vital to the project's success. If they didn't exist, it's not a guarantee that you could have replaced all of them -- the project may have simply failed.
What you do in life, echoes in eternity.
But to address your question: people 'take the limit' and argue that life is just meaningless in every way all the time. If it were true, you shouldn't be bothered to make that effort in the first place. Obviously your actions matter to other people by the sheer virtue of the fact that you're optimizing for it. if you weren't, you wouldn't have bothered to ask the question.
Sometimes life is what you actually do, not merely what you think.