An article appeared recently on dnainfo.com about a World War I era monument, sponsored by the Chicago Council of Gold Star Mothers, being dismantled and "stored" because no one can figure out exactly what it commemorated, or even what it originally looked like. It's just a pile of rocks now, used as a toilet by those waiting for the bus.
That's summarizes the idea of legacy to me. Anyone trying to tell you it's something you can control is either selling you something or trying to justify their life's decisions (often to themselves).
Even the most famous humans are rarely remembered more than 100 years or so. No humans will be around to be remembered when the next large asteroid hits (or super volcano, etc.). Mass extinctions are real events that happen with regularity on this planet.
Even if those things didn't occur and humans somehow live for millions or billions of years, scattered throughout the universe, the universe itself has an expiration date.
Legacy happens outside of your control, and always expires.
Don't think of that as a downer. It's a wonderful equalizer.
It's also a great reminder that all we really have is now.