Just because it's the last adventure doesn't mean it's the last game in the series. They're talking about last adventure in the Uncharted timeline.
Nathan Drake could have went on many other adventures that we're not aware of. It all depends on how many of these adventures the developers create. They could even make a prequel with Sully and Drake after the events of their first meeting as seen in Uncharted 3. There are endless possibilities of new Uncharted games with Nathan Drake and Victor Sullivan. There are near endless possibilities even with new characters like Cutter, Elena, and Chloe as well.
Who knows. In a single year, Nathan Drake could find dozens of priceless treasures with a quirk behind them that they can't let fall into the wrong hands so they don't make the discovery in hopes that no one else finds them.
