I'm going to have to disagree. I was going to vote for Robinson but I thought of it more and realized that all four of those people saved the franchise one way or another.
First Drossos was the driving force behind the very existence of the franchise. He helped secure the original ABA franchise, he went out and got Gervin, fought to keep that trade valid and then basically refused to be ignored and got the Spurs into the NBA. He definitely deserves a lot of credit and really shouldn't fall out of the top four in this list.
Then you have Gervin who brought the entertainment value to the Spurs and put them on the map. Without Gervin, the Spurs likely don't draw the crowds and then they don't have the financial stability to last. Without Gervin, the chances San Antonio is let into the NBA are probably close to zero. San Antonio needed a superstar to survive and he definitely was just that.
Robinson has been covered extensively in this thread and he is due of all the praise he has been given. He came to San Antonio, saw the rich tradition of the franchise and realized that if he didn't sign with the Spurs, the Spurs would have died. Out of pure and possibly unfounded loyalty he kept the Spurs afloat numerous times when he could have easily turned his back and gone on to greener surroundings. But he decided that keeping the Spurs alive was more important than money, fame and championship. And even today, he remains a big part of everything.
But to say Duncan didn't save the Spurs isn't accurate. If the Spurs don't win the 1999 championship, the SBC Center never gets built and the Spurs leave. The difference with Duncan and Robinson is that Robinson overtly saved the franchise ... while Duncan just so happened to save the franchise on his way to winning championships. Robinson was dedicated to San Antonio, the Spurs and keeping the legacy. Duncan is dedicated to winning championships. But even though Duncan has different motives, the result of saving the franchise was the same. If the Spurs didn't turn into a championship franchise when they did, they were on the chopping block to be sent packing. All the hard work from Drossos, Gervin and Robinson would have been lost.
Now if you consider all four of those individuals as franchise savers, I think Duncan barely takes the lead over Robinson. By a very, very narrow margin. And really, it's not a fair playing field because Robinson suffered perhaps the worst run organization and worst set of teammates around him during his prime of any superstar in NBA history. On the other hand, Duncan probably stepped into one of the three or four best situations of anyone in NBA history. But sports is results driven and Duncan has helped deliver all four championships and his passive aggressive demand of continued winning has kept the Spurs at a high level for a long amount of time. Duncan definitely has received a tremendous amount of help throughout the years and lucked into the situation he fell into but he has taken the Spurs from a storybook franchise to a championship franchise that will one day be remembered for this dominant stretch we are currently witnessing.
It is really damn close but I have to put Duncan as number one, with a sincere tip of the hat to the other three individuals who made it possible for the Spurs to become what they are today.