I want to know the odds we have of honestly keeping him a Nat his entire career
The entire career thing isn't realistic. Look at the best player of the last 10 years: Albert Pujols. He brought St. Louis 10 amazing seasons and 2 World Series titles, and then he went to Anaheim. Or look at a guy like Ken Griffey, with the (trade) interlude in Cincy. Even if we signed Harper through age 32 and won three titles with him, that doesn't mean we will (or should) keep him to age 42. How many players are like that? Since I was born, only Cal Ripken and Mariano Rivera. No-one else.