> 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.