They use a large sample of baseball players and track their stats (plate discipline, sprint speed, bat speed, etc) across ages to look at which skills deteriorate less than others and then assign that to the skills of the players. And you are right, there is no way to say definitively x will happen to a player, but you can look at tendencies across a population to help inform likelihoods.
And if any team is about to spend a significant investment I guarantee they are doing some projecting.